Node.js util module | API Reference | Bun

The util.inspect() method returns a string representation of object that is intended for debugging. The output of util.inspect may change at any time and should not be depended upon programmatically. Additional options may be passed that alter the result. util.inspect() will use the constructor's name and/or Symbol.toStringTag property to make an identifiable tag for an inspected value.

class Foo {
get [Symbol.toStringTag]() {
return 'bar';
}
}

class Bar {}

const baz = Object.create(null, { [Symbol.toStringTag]: { value: 'foo' } });

util.inspect(new Foo()); // 'Foo [bar] {}'
util.inspect(new Bar()); // 'Bar {}'
util.inspect(baz); // '[foo] {}'

Circular references point to their anchor by using a reference index:

import { inspect } from 'node:util';

const obj = {};
obj.a = [obj];
obj.b = {};
obj.b.inner = obj.b;
obj.b.obj = obj;

console.log(inspect(obj));
// {
// a: [ [Circular *1] ],
// b: { inner: [Circular *2], obj: [Circular *1] }
// }

The following example inspects all properties of the util object:

import util from 'node:util';

console.log(util.inspect(util, { showHidden: true, depth: null }));

The following example highlights the effect of the compact option:

import { inspect } from 'node:util';

const o = {
a: [1, 2, [[
'Lorem ipsum dolor sit amet,\nconsectetur adipiscing elit, sed do ' +
'eiusmod \ntempor incididunt ut labore et dolore magna aliqua.',
'test',
'foo']], 4],
b: new Map([['za', 1], ['zb', 'test']]),
};
console.log(inspect(o, { compact: true, depth: 5, breakLength: 80 }));

// { a:
// [ 1,
// 2,
// [ [ 'Lorem ipsum dolor sit amet,\nconsectetur [...]', // A long line
// 'test',
// 'foo' ] ],
// 4 ],
// b: Map(2) { 'za' => 1, 'zb' => 'test' } }

// Setting `compact` to false or an integer creates more reader friendly output.
console.log(inspect(o, { compact: false, depth: 5, breakLength: 80 }));

// {
// a: [
// 1,
// 2,
// [
// [
// 'Lorem ipsum dolor sit amet,\n' +
// 'consectetur adipiscing elit, sed do eiusmod \n' +
// 'tempor incididunt ut labore et dolore magna aliqua.',
// 'test',
// 'foo'
// ]
// ],
// 4
// ],
// b: Map(2) {
// 'za' => 1,
// 'zb' => 'test'
// }
// }

// Setting `breakLength` to e.g. 150 will print the "Lorem ipsum" text in a
// single line.

The showHidden option allows WeakMap and WeakSet entries to be inspected. If there are more entries than maxArrayLength, there is no guarantee which entries are displayed. That means retrieving the same WeakSet entries twice may result in different output. Furthermore, entries with no remaining strong references may be garbage collected at any time.

import { inspect } from 'node:util';

const obj = { a: 1 };
const obj2 = { b: 2 };
const weakSet = new WeakSet([obj, obj2]);

console.log(inspect(weakSet, { showHidden: true }));
// WeakSet { { a: 1 }, { b: 2 } }

The sorted option ensures that an object's property insertion order does not impact the result of util.inspect().

import { inspect } from 'node:util';
import assert from 'node:assert';

const o1 = {
b: [2, 3, 1],
a: '`a` comes before `b`',
c: new Set([2, 3, 1]),
};
console.log(inspect(o1, { sorted: true }));
// { a: '`a` comes before `b`', b: [ 2, 3, 1 ], c: Set(3) { 1, 2, 3 } }
console.log(inspect(o1, { sorted: (a, b) => b.localeCompare(a) }));
// { c: Set(3) { 3, 2, 1 }, b: [ 2, 3, 1 ], a: '`a` comes before `b`' }

const o2 = {
c: new Set([2, 1, 3]),
a: '`a` comes before `b`',
b: [2, 3, 1],
};
assert.strict.equal(
inspect(o1, { sorted: true }),
inspect(o2, { sorted: true }),
);

The numericSeparator option adds an underscore every three digits to all numbers.

import { inspect } from 'node:util';

const thousand = 1000;
const million = 1000000;
const bigNumber = 123456789n;
const bigDecimal = 1234.12345;

console.log(inspect(thousand, { numericSeparator: true }));
// 1_000
console.log(inspect(million, { numericSeparator: true }));
// 1_000_000
console.log(inspect(bigNumber, { numericSeparator: true }));
// 123_456_789n
console.log(inspect(bigDecimal, { numericSeparator: true }));
// 1_234.123_45

util.inspect() is a synchronous method intended for debugging. Its maximum output length is approximately 128 MiB. Inputs that result in longer output will be truncated.

Any JavaScript primitive or Object.

The representation of object.

The util.inspect() method returns a string representation of object that is intended for debugging. The output of util.inspect may change at any time and should not be depended upon programmatically. Additional options may be passed that alter the result. util.inspect() will use the constructor's name and/or Symbol.toStringTag property to make an identifiable tag for an inspected value.

class Foo {
get [Symbol.toStringTag]() {
return 'bar';
}
}

class Bar {}

const baz = Object.create(null, { [Symbol.toStringTag]: { value: 'foo' } });

util.inspect(new Foo()); // 'Foo [bar] {}'
util.inspect(new Bar()); // 'Bar {}'
util.inspect(baz); // '[foo] {}'

Circular references point to their anchor by using a reference index:

import { inspect } from 'node:util';

const obj = {};
obj.a = [obj];
obj.b = {};
obj.b.inner = obj.b;
obj.b.obj = obj;

console.log(inspect(obj));
// {
// a: [ [Circular *1] ],
// b: { inner: [Circular *2], obj: [Circular *1] }
// }

The following example inspects all properties of the util object:

import util from 'node:util';

console.log(util.inspect(util, { showHidden: true, depth: null }));

The following example highlights the effect of the compact option:

import { inspect } from 'node:util';

const o = {
a: [1, 2, [[
'Lorem ipsum dolor sit amet,\nconsectetur adipiscing elit, sed do ' +
'eiusmod \ntempor incididunt ut labore et dolore magna aliqua.',
'test',
'foo']], 4],
b: new Map([['za', 1], ['zb', 'test']]),
};
console.log(inspect(o, { compact: true, depth: 5, breakLength: 80 }));

// { a:
// [ 1,
// 2,
// [ [ 'Lorem ipsum dolor sit amet,\nconsectetur [...]', // A long line
// 'test',
// 'foo' ] ],
// 4 ],
// b: Map(2) { 'za' => 1, 'zb' => 'test' } }

// Setting `compact` to false or an integer creates more reader friendly output.
console.log(inspect(o, { compact: false, depth: 5, breakLength: 80 }));

// {
// a: [
// 1,
// 2,
// [
// [
// 'Lorem ipsum dolor sit amet,\n' +
// 'consectetur adipiscing elit, sed do eiusmod \n' +
// 'tempor incididunt ut labore et dolore magna aliqua.',
// 'test',
// 'foo'
// ]
// ],
// 4
// ],
// b: Map(2) {
// 'za' => 1,
// 'zb' => 'test'
// }
// }

// Setting `breakLength` to e.g. 150 will print the "Lorem ipsum" text in a
// single line.

The showHidden option allows WeakMap and WeakSet entries to be inspected. If there are more entries than maxArrayLength, there is no guarantee which entries are displayed. That means retrieving the same WeakSet entries twice may result in different output. Furthermore, entries with no remaining strong references may be garbage collected at any time.

import { inspect } from 'node:util';

const obj = { a: 1 };
const obj2 = { b: 2 };
const weakSet = new WeakSet([obj, obj2]);

console.log(inspect(weakSet, { showHidden: true }));
// WeakSet { { a: 1 }, { b: 2 } }

The sorted option ensures that an object's property insertion order does not impact the result of util.inspect().

import { inspect } from 'node:util';
import assert from 'node:assert';

const o1 = {
b: [2, 3, 1],
a: '`a` comes before `b`',
c: new Set([2, 3, 1]),
};
console.log(inspect(o1, { sorted: true }));
// { a: '`a` comes before `b`', b: [ 2, 3, 1 ], c: Set(3) { 1, 2, 3 } }
console.log(inspect(o1, { sorted: (a, b) => b.localeCompare(a) }));
// { c: Set(3) { 3, 2, 1 }, b: [ 2, 3, 1 ], a: '`a` comes before `b`' }

const o2 = {
c: new Set([2, 1, 3]),
a: '`a` comes before `b`',
b: [2, 3, 1],
};
assert.strict.equal(
inspect(o1, { sorted: true }),
inspect(o2, { sorted: true }),
);

The numericSeparator option adds an underscore every three digits to all numbers.

import { inspect } from 'node:util';

const thousand = 1000;
const million = 1000000;
const bigNumber = 123456789n;
const bigDecimal = 1234.12345;

console.log(inspect(thousand, { numericSeparator: true }));
// 1_000
console.log(inspect(million, { numericSeparator: true }));
// 1_000_000
console.log(inspect(bigNumber, { numericSeparator: true }));
// 123_456_789n
console.log(inspect(bigDecimal, { numericSeparator: true }));
// 1_234.123_45

util.inspect() is a synchronous method intended for debugging. Its maximum output length is approximately 128 MiB. Inputs that result in longer output will be truncated.

Any JavaScript primitive or Object.

The representation of object.

Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);
promisifiedStat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});

Or, equivalently using async functions:

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);

async function callStat() {
const stats = await promisifiedStat('.');
console.log(`This directory is owned by ${stats.uid}`);
}

callStat();

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:

import { promisify } from 'node:util';

class Foo {
constructor() {
this.a = 42;
}

bar(callback) {
callback(null, this.a);
}
}

const foo = new Foo();

const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'

Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);
promisifiedStat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});

Or, equivalently using async functions:

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);

async function callStat() {
const stats = await promisifiedStat('.');
console.log(`This directory is owned by ${stats.uid}`);
}

callStat();

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:

import { promisify } from 'node:util';

class Foo {
constructor() {
this.a = 42;
}

bar(callback) {
callback(null, this.a);
}
}

const foo = new Foo();

const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'

Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);
promisifiedStat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});

Or, equivalently using async functions:

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);

async function callStat() {
const stats = await promisifiedStat('.');
console.log(`This directory is owned by ${stats.uid}`);
}

callStat();

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:

import { promisify } from 'node:util';

class Foo {
constructor() {
this.a = 42;
}

bar(callback) {
callback(null, this.a);
}
}

const foo = new Foo();

const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'

Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);
promisifiedStat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});

Or, equivalently using async functions:

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);

async function callStat() {
const stats = await promisifiedStat('.');
console.log(`This directory is owned by ${stats.uid}`);
}

callStat();

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:

import { promisify } from 'node:util';

class Foo {
constructor() {
this.a = 42;
}

bar(callback) {
callback(null, this.a);
}
}

const foo = new Foo();

const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'

Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);
promisifiedStat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});

Or, equivalently using async functions:

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);

async function callStat() {
const stats = await promisifiedStat('.');
console.log(`This directory is owned by ${stats.uid}`);
}

callStat();

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:

import { promisify } from 'node:util';

class Foo {
constructor() {
this.a = 42;
}

bar(callback) {
callback(null, this.a);
}
}

const foo = new Foo();

const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'

Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);
promisifiedStat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});

Or, equivalently using async functions:

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);

async function callStat() {
const stats = await promisifiedStat('.');
console.log(`This directory is owned by ${stats.uid}`);
}

callStat();

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:

import { promisify } from 'node:util';

class Foo {
constructor() {
this.a = 42;
}

bar(callback) {
callback(null, this.a);
}
}

const foo = new Foo();

const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'

Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);
promisifiedStat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});

Or, equivalently using async functions:

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);

async function callStat() {
const stats = await promisifiedStat('.');
console.log(`This directory is owned by ${stats.uid}`);
}

callStat();

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:

import { promisify } from 'node:util';

class Foo {
constructor() {
this.a = 42;
}

bar(callback) {
callback(null, this.a);
}
}

const foo = new Foo();

const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'

Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);
promisifiedStat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});

Or, equivalently using async functions:

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);

async function callStat() {
const stats = await promisifiedStat('.');
console.log(`This directory is owned by ${stats.uid}`);
}

callStat();

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:

import { promisify } from 'node:util';

class Foo {
constructor() {
this.a = 42;
}

bar(callback) {
callback(null, this.a);
}
}

const foo = new Foo();

const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'

Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);
promisifiedStat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});

Or, equivalently using async functions:

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);

async function callStat() {
const stats = await promisifiedStat('.');
console.log(`This directory is owned by ${stats.uid}`);
}

callStat();

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:

import { promisify } from 'node:util';

class Foo {
constructor() {
this.a = 42;
}

bar(callback) {
callback(null, this.a);
}
}

const foo = new Foo();

const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'

Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);
promisifiedStat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});

Or, equivalently using async functions:

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);

async function callStat() {
const stats = await promisifiedStat('.');
console.log(`This directory is owned by ${stats.uid}`);
}

callStat();

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:

import { promisify } from 'node:util';

class Foo {
constructor() {
this.a = 42;
}

bar(callback) {
callback(null, this.a);
}
}

const foo = new Foo();

const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'

Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);
promisifiedStat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});

Or, equivalently using async functions:

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);

async function callStat() {
const stats = await promisifiedStat('.');
console.log(`This directory is owned by ${stats.uid}`);
}

callStat();

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:

import { promisify } from 'node:util';

class Foo {
constructor() {
this.a = 42;
}

bar(callback) {
callback(null, this.a);
}
}

const foo = new Foo();

const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'

Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);
promisifiedStat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});

Or, equivalently using async functions:

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);

async function callStat() {
const stats = await promisifiedStat('.');
console.log(`This directory is owned by ${stats.uid}`);
}

callStat();

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:

import { promisify } from 'node:util';

class Foo {
constructor() {
this.a = 42;
}

bar(callback) {
callback(null, this.a);
}
}

const foo = new Foo();

const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'

Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);
promisifiedStat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});

Or, equivalently using async functions:

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);

async function callStat() {
const stats = await promisifiedStat('.');
console.log(`This directory is owned by ${stats.uid}`);
}

callStat();

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:

import { promisify } from 'node:util';

class Foo {
constructor() {
this.a = 42;
}

bar(callback) {
callback(null, this.a);
}
}

const foo = new Foo();

const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'

Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);
promisifiedStat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});

Or, equivalently using async functions:

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);

async function callStat() {
const stats = await promisifiedStat('.');
console.log(`This directory is owned by ${stats.uid}`);
}

callStat();

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:

import { promisify } from 'node:util';

class Foo {
constructor() {
this.a = 42;
}

bar(callback) {
callback(null, this.a);
}
}

const foo = new Foo();

const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'

That can be used to declare custom promisified variants of functions.

The MIMEParams API provides read and write access to the parameters of a MIMEType.

Returns an iterator over each of the name-value pairs in the parameters.

Remove all name-value pairs whose name is name.

Returns an iterator over each of the name-value pairs in the parameters. Each item of the iterator is a JavaScript Array. The first item of the array is the name, the second item of the array is the value.

Returns the value of the first name-value pair whose name is name. If there are no such pairs, null is returned.

or null if there is no name-value pair with the given name.

Returns true if there is at least one name-value pair whose name is name.

Returns an iterator over the names of each name-value pair.

import { MIMEType } from 'node:util';

const { params } = new MIMEType('text/plain;foo=0;bar=1');
for (const name of params.keys()) {
console.log(name);
}
// Prints:
// foo
// bar

Sets the value in the MIMEParams object associated with name to value. If there are any pre-existing name-value pairs whose names are name, set the first such pair's value to value.

import { MIMEType } from 'node:util';

const { params } = new MIMEType('text/plain;foo=0;bar=1');
params.set('foo', 'def');
params.set('baz', 'xyz');
console.log(params.toString());
// Prints: foo=def;bar=1;baz=xyz

Returns an iterator over the values of each name-value pair.

An implementation of the MIMEType class.

In accordance with browser conventions, all properties of MIMEType objects are implemented as getters and setters on the class prototype, rather than as data properties on the object itself.

A MIME string is a structured string containing multiple meaningful components. When parsed, a MIMEType object is returned containing properties for each of these components.

Gets the essence of the MIME. This property is read only. Use mime.type or mime.subtype to alter the MIME.

import { MIMEType } from 'node:util';

const myMIME = new MIMEType('text/javascript;key=value');
console.log(myMIME.essence);
// Prints: text/javascript
myMIME.type = 'application';
console.log(myMIME.essence);
// Prints: application/javascript
console.log(String(myMIME));
// Prints: application/javascript;key=value

Gets the MIMEParams object representing the parameters of the MIME. This property is read-only. See MIMEParams documentation for details.

Gets and sets the subtype portion of the MIME.

import { MIMEType } from 'node:util';

const myMIME = new MIMEType('text/ecmascript');
console.log(myMIME.subtype);
// Prints: ecmascript
myMIME.subtype = 'javascript';
console.log(myMIME.subtype);
// Prints: javascript
console.log(String(myMIME));
// Prints: text/javascript

Gets and sets the type portion of the MIME.

import { MIMEType } from 'node:util';

const myMIME = new MIMEType('text/javascript');
console.log(myMIME.type);
// Prints: text
myMIME.type = 'application';
console.log(myMIME.type);
// Prints: application
console.log(String(myMIME));
// Prints: application/javascript

The toString() method on the MIMEType object returns the serialized MIME.

Because of the need for standard compliance, this method does not allow users to customize the serialization process of the MIME.

Listens to abort event on the provided signal and returns a promise that resolves when the signal is aborted. If resource is provided, it weakly references the operation's associated object, so if resource is garbage collected before the signal aborts, then returned promise shall remain pending. This prevents memory leaks in long-running or non-cancelable operations.

import { aborted } from 'node:util';

// Obtain an object with an abortable signal, like a custom resource or operation.
const dependent = obtainSomethingAbortable();

// Pass `dependent` as the resource, indicating the promise should only resolve
// if `dependent` is still in memory when the signal is aborted.
aborted(dependent.signal, dependent).then(() => {
// This code runs when `dependent` is aborted.
console.log('Dependent resource was aborted.');
});

// Simulate an event that triggers the abort.
dependent.on('event', () => {
dependent.abort(); // This will cause the `aborted` promise to resolve.
});

Any non-null object tied to the abortable operation and held weakly. If resource is garbage collected before the signal aborts, the promise remains pending, allowing Node.js to stop tracking it. This helps prevent memory leaks in long-running or non-cancelable operations.

Takes an async function (or a function that returns a Promise) and returns a function following the error-first callback style, i.e. taking an (err, value) => ... callback as the last argument. In the callback, the first argument will be the rejection reason (or null if the Promise resolved), and the second argument will be the resolved value.

import { callbackify } from 'node:util';

async function fn() {
return 'hello world';
}
const callbackFunction = callbackify(fn);

callbackFunction((err, ret) => {
if (err) throw err;
console.log(ret);
});

Will print:

hello world

The callback is executed asynchronously, and will have a limited stack trace. If the callback throws, the process will emit an 'uncaughtException' event, and if not handled will exit.

Since null has a special meaning as the first argument to a callback, if a wrapped function rejects a Promise with a falsy value as a reason, the value is wrapped in an Error with the original value stored in a field named reason.

function fn() {
return Promise.reject(null);
}
const callbackFunction = util.callbackify(fn);

callbackFunction((err, ret) => {
// When the Promise was rejected with `null` it is wrapped with an Error and
// the original value is stored in `reason`.
err && Object.hasOwn(err, 'reason') && err.reason === null; // true
});

An async function

a callback style function

Takes an async function (or a function that returns a Promise) and returns a function following the error-first callback style, i.e. taking an (err, value) => ... callback as the last argument. In the callback, the first argument will be the rejection reason (or null if the Promise resolved), and the second argument will be the resolved value.

import { callbackify } from 'node:util';

async function fn() {
return 'hello world';
}
const callbackFunction = callbackify(fn);

callbackFunction((err, ret) => {
if (err) throw err;
console.log(ret);
});

Will print:

hello world

The callback is executed asynchronously, and will have a limited stack trace. If the callback throws, the process will emit an 'uncaughtException' event, and if not handled will exit.

Since null has a special meaning as the first argument to a callback, if a wrapped function rejects a Promise with a falsy value as a reason, the value is wrapped in an Error with the original value stored in a field named reason.

function fn() {
return Promise.reject(null);
}
const callbackFunction = util.callbackify(fn);

callbackFunction((err, ret) => {
// When the Promise was rejected with `null` it is wrapped with an Error and
// the original value is stored in `reason`.
err && Object.hasOwn(err, 'reason') && err.reason === null; // true
});

An async function

a callback style function

Takes an async function (or a function that returns a Promise) and returns a function following the error-first callback style, i.e. taking an (err, value) => ... callback as the last argument. In the callback, the first argument will be the rejection reason (or null if the Promise resolved), and the second argument will be the resolved value.

import { callbackify } from 'node:util';

async function fn() {
return 'hello world';
}
const callbackFunction = callbackify(fn);

callbackFunction((err, ret) => {
if (err) throw err;
console.log(ret);
});

Will print:

hello world

The callback is executed asynchronously, and will have a limited stack trace. If the callback throws, the process will emit an 'uncaughtException' event, and if not handled will exit.

Since null has a special meaning as the first argument to a callback, if a wrapped function rejects a Promise with a falsy value as a reason, the value is wrapped in an Error with the original value stored in a field named reason.

function fn() {
return Promise.reject(null);
}
const callbackFunction = util.callbackify(fn);

callbackFunction((err, ret) => {
// When the Promise was rejected with `null` it is wrapped with an Error and
// the original value is stored in `reason`.
err && Object.hasOwn(err, 'reason') && err.reason === null; // true
});

An async function

a callback style function

Takes an async function (or a function that returns a Promise) and returns a function following the error-first callback style, i.e. taking an (err, value) => ... callback as the last argument. In the callback, the first argument will be the rejection reason (or null if the Promise resolved), and the second argument will be the resolved value.

import { callbackify } from 'node:util';

async function fn() {
return 'hello world';
}
const callbackFunction = callbackify(fn);

callbackFunction((err, ret) => {
if (err) throw err;
console.log(ret);
});

Will print:

hello world

The callback is executed asynchronously, and will have a limited stack trace. If the callback throws, the process will emit an 'uncaughtException' event, and if not handled will exit.

Since null has a special meaning as the first argument to a callback, if a wrapped function rejects a Promise with a falsy value as a reason, the value is wrapped in an Error with the original value stored in a field named reason.

function fn() {
return Promise.reject(null);
}
const callbackFunction = util.callbackify(fn);

callbackFunction((err, ret) => {
// When the Promise was rejected with `null` it is wrapped with an Error and
// the original value is stored in `reason`.
err && Object.hasOwn(err, 'reason') && err.reason === null; // true
});

An async function

a callback style function

Takes an async function (or a function that returns a Promise) and returns a function following the error-first callback style, i.e. taking an (err, value) => ... callback as the last argument. In the callback, the first argument will be the rejection reason (or null if the Promise resolved), and the second argument will be the resolved value.

import { callbackify } from 'node:util';

async function fn() {
return 'hello world';
}
const callbackFunction = callbackify(fn);

callbackFunction((err, ret) => {
if (err) throw err;
console.log(ret);
});

Will print:

hello world

The callback is executed asynchronously, and will have a limited stack trace. If the callback throws, the process will emit an 'uncaughtException' event, and if not handled will exit.

Since null has a special meaning as the first argument to a callback, if a wrapped function rejects a Promise with a falsy value as a reason, the value is wrapped in an Error with the original value stored in a field named reason.

function fn() {
return Promise.reject(null);
}
const callbackFunction = util.callbackify(fn);

callbackFunction((err, ret) => {
// When the Promise was rejected with `null` it is wrapped with an Error and
// the original value is stored in `reason`.
err && Object.hasOwn(err, 'reason') && err.reason === null; // true
});

An async function

a callback style function

Takes an async function (or a function that returns a Promise) and returns a function following the error-first callback style, i.e. taking an (err, value) => ... callback as the last argument. In the callback, the first argument will be the rejection reason (or null if the Promise resolved), and the second argument will be the resolved value.

import { callbackify } from 'node:util';

async function fn() {
return 'hello world';
}
const callbackFunction = callbackify(fn);

callbackFunction((err, ret) => {
if (err) throw err;
console.log(ret);
});

Will print:

hello world

The callback is executed asynchronously, and will have a limited stack trace. If the callback throws, the process will emit an 'uncaughtException' event, and if not handled will exit.

Since null has a special meaning as the first argument to a callback, if a wrapped function rejects a Promise with a falsy value as a reason, the value is wrapped in an Error with the original value stored in a field named reason.

function fn() {
return Promise.reject(null);
}
const callbackFunction = util.callbackify(fn);

callbackFunction((err, ret) => {
// When the Promise was rejected with `null` it is wrapped with an Error and
// the original value is stored in `reason`.
err && Object.hasOwn(err, 'reason') && err.reason === null; // true
});

An async function

a callback style function

Takes an async function (or a function that returns a Promise) and returns a function following the error-first callback style, i.e. taking an (err, value) => ... callback as the last argument. In the callback, the first argument will be the rejection reason (or null if the Promise resolved), and the second argument will be the resolved value.

import { callbackify } from 'node:util';

async function fn() {
return 'hello world';
}
const callbackFunction = callbackify(fn);

callbackFunction((err, ret) => {
if (err) throw err;
console.log(ret);
});

Will print:

hello world

The callback is executed asynchronously, and will have a limited stack trace. If the callback throws, the process will emit an 'uncaughtException' event, and if not handled will exit.

Since null has a special meaning as the first argument to a callback, if a wrapped function rejects a Promise with a falsy value as a reason, the value is wrapped in an Error with the original value stored in a field named reason.

function fn() {
return Promise.reject(null);
}
const callbackFunction = util.callbackify(fn);

callbackFunction((err, ret) => {
// When the Promise was rejected with `null` it is wrapped with an Error and
// the original value is stored in `reason`.
err && Object.hasOwn(err, 'reason') && err.reason === null; // true
});

An async function

a callback style function

Takes an async function (or a function that returns a Promise) and returns a function following the error-first callback style, i.e. taking an (err, value) => ... callback as the last argument. In the callback, the first argument will be the rejection reason (or null if the Promise resolved), and the second argument will be the resolved value.

import { callbackify } from 'node:util';

async function fn() {
return 'hello world';
}
const callbackFunction = callbackify(fn);

callbackFunction((err, ret) => {
if (err) throw err;
console.log(ret);
});

Will print:

hello world

The callback is executed asynchronously, and will have a limited stack trace. If the callback throws, the process will emit an 'uncaughtException' event, and if not handled will exit.

Since null has a special meaning as the first argument to a callback, if a wrapped function rejects a Promise with a falsy value as a reason, the value is wrapped in an Error with the original value stored in a field named reason.

function fn() {
return Promise.reject(null);
}
const callbackFunction = util.callbackify(fn);

callbackFunction((err, ret) => {
// When the Promise was rejected with `null` it is wrapped with an Error and
// the original value is stored in `reason`.
err && Object.hasOwn(err, 'reason') && err.reason === null; // true
});

An async function

a callback style function

Takes an async function (or a function that returns a Promise) and returns a function following the error-first callback style, i.e. taking an (err, value) => ... callback as the last argument. In the callback, the first argument will be the rejection reason (or null if the Promise resolved), and the second argument will be the resolved value.

import { callbackify } from 'node:util';

async function fn() {
return 'hello world';
}
const callbackFunction = callbackify(fn);

callbackFunction((err, ret) => {
if (err) throw err;
console.log(ret);
});

Will print:

hello world

The callback is executed asynchronously, and will have a limited stack trace. If the callback throws, the process will emit an 'uncaughtException' event, and if not handled will exit.

Since null has a special meaning as the first argument to a callback, if a wrapped function rejects a Promise with a falsy value as a reason, the value is wrapped in an Error with the original value stored in a field named reason.

function fn() {
return Promise.reject(null);
}
const callbackFunction = util.callbackify(fn);

callbackFunction((err, ret) => {
// When the Promise was rejected with `null` it is wrapped with an Error and
// the original value is stored in `reason`.
err && Object.hasOwn(err, 'reason') && err.reason === null; // true
});

An async function

a callback style function

Takes an async function (or a function that returns a Promise) and returns a function following the error-first callback style, i.e. taking an (err, value) => ... callback as the last argument. In the callback, the first argument will be the rejection reason (or null if the Promise resolved), and the second argument will be the resolved value.

import { callbackify } from 'node:util';

async function fn() {
return 'hello world';
}
const callbackFunction = callbackify(fn);

callbackFunction((err, ret) => {
if (err) throw err;
console.log(ret);
});

Will print:

hello world

The callback is executed asynchronously, and will have a limited stack trace. If the callback throws, the process will emit an 'uncaughtException' event, and if not handled will exit.

Since null has a special meaning as the first argument to a callback, if a wrapped function rejects a Promise with a falsy value as a reason, the value is wrapped in an Error with the original value stored in a field named reason.

function fn() {
return Promise.reject(null);
}
const callbackFunction = util.callbackify(fn);

callbackFunction((err, ret) => {
// When the Promise was rejected with `null` it is wrapped with an Error and
// the original value is stored in `reason`.
err && Object.hasOwn(err, 'reason') && err.reason === null; // true
});

An async function

a callback style function

Takes an async function (or a function that returns a Promise) and returns a function following the error-first callback style, i.e. taking an (err, value) => ... callback as the last argument. In the callback, the first argument will be the rejection reason (or null if the Promise resolved), and the second argument will be the resolved value.

import { callbackify } from 'node:util';

async function fn() {
return 'hello world';
}
const callbackFunction = callbackify(fn);

callbackFunction((err, ret) => {
if (err) throw err;
console.log(ret);
});

Will print:

hello world

The callback is executed asynchronously, and will have a limited stack trace. If the callback throws, the process will emit an 'uncaughtException' event, and if not handled will exit.

Since null has a special meaning as the first argument to a callback, if a wrapped function rejects a Promise with a falsy value as a reason, the value is wrapped in an Error with the original value stored in a field named reason.

function fn() {
return Promise.reject(null);
}
const callbackFunction = util.callbackify(fn);

callbackFunction((err, ret) => {
// When the Promise was rejected with `null` it is wrapped with an Error and
// the original value is stored in `reason`.
err && Object.hasOwn(err, 'reason') && err.reason === null; // true
});

An async function

a callback style function

Takes an async function (or a function that returns a Promise) and returns a function following the error-first callback style, i.e. taking an (err, value) => ... callback as the last argument. In the callback, the first argument will be the rejection reason (or null if the Promise resolved), and the second argument will be the resolved value.

import { callbackify } from 'node:util';

async function fn() {
return 'hello world';
}
const callbackFunction = callbackify(fn);

callbackFunction((err, ret) => {
if (err) throw err;
console.log(ret);
});

Will print:

hello world

The callback is executed asynchronously, and will have a limited stack trace. If the callback throws, the process will emit an 'uncaughtException' event, and if not handled will exit.

Since null has a special meaning as the first argument to a callback, if a wrapped function rejects a Promise with a falsy value as a reason, the value is wrapped in an Error with the original value stored in a field named reason.

function fn() {
return Promise.reject(null);
}
const callbackFunction = util.callbackify(fn);

callbackFunction((err, ret) => {
// When the Promise was rejected with `null` it is wrapped with an Error and
// the original value is stored in `reason`.
err && Object.hasOwn(err, 'reason') && err.reason === null; // true
});

An async function

a callback style function

Takes an async function (or a function that returns a Promise) and returns a function following the error-first callback style, i.e. taking an (err, value) => ... callback as the last argument. In the callback, the first argument will be the rejection reason (or null if the Promise resolved), and the second argument will be the resolved value.

import { callbackify } from 'node:util';

async function fn() {
return 'hello world';
}
const callbackFunction = callbackify(fn);

callbackFunction((err, ret) => {
if (err) throw err;
console.log(ret);
});

Will print:

hello world

The callback is executed asynchronously, and will have a limited stack trace. If the callback throws, the process will emit an 'uncaughtException' event, and if not handled will exit.

Since null has a special meaning as the first argument to a callback, if a wrapped function rejects a Promise with a falsy value as a reason, the value is wrapped in an Error with the original value stored in a field named reason.

function fn() {
return Promise.reject(null);
}
const callbackFunction = util.callbackify(fn);

callbackFunction((err, ret) => {
// When the Promise was rejected with `null` it is wrapped with an Error and
// the original value is stored in `reason`.
err && Object.hasOwn(err, 'reason') && err.reason === null; // true
});

An async function

a callback style function

Takes an async function (or a function that returns a Promise) and returns a function following the error-first callback style, i.e. taking an (err, value) => ... callback as the last argument. In the callback, the first argument will be the rejection reason (or null if the Promise resolved), and the second argument will be the resolved value.

import { callbackify } from 'node:util';

async function fn() {
return 'hello world';
}
const callbackFunction = callbackify(fn);

callbackFunction((err, ret) => {
if (err) throw err;
console.log(ret);
});

Will print:

hello world

The callback is executed asynchronously, and will have a limited stack trace. If the callback throws, the process will emit an 'uncaughtException' event, and if not handled will exit.

Since null has a special meaning as the first argument to a callback, if a wrapped function rejects a Promise with a falsy value as a reason, the value is wrapped in an Error with the original value stored in a field named reason.

function fn() {
return Promise.reject(null);
}
const callbackFunction = util.callbackify(fn);

callbackFunction((err, ret) => {
// When the Promise was rejected with `null` it is wrapped with an Error and
// the original value is stored in `reason`.
err && Object.hasOwn(err, 'reason') && err.reason === null; // true
});

An async function

a callback style function

The util.convertProcessSignalToExitCode() method converts a signal name to its corresponding POSIX exit code. Following the POSIX standard, the exit code for a process terminated by a signal is calculated as 128 + signal number.

If signal is not a valid signal name, then an error will be thrown. See signal(7) for a list of valid signals.

import { convertProcessSignalToExitCode } from 'node:util';

console.log(convertProcessSignalToExitCode('SIGTERM')); // 143 (128 + 15)
console.log(convertProcessSignalToExitCode('SIGKILL')); // 137 (128 + 9)

This is particularly useful when working with processes to determine the exit code based on the signal that terminated the process.

A signal name (e.g. 'SIGTERM')

The exit code corresponding to signal

The util.debuglog() method is used to create a function that conditionally writes debug messages to stderr based on the existence of the NODE_DEBUG environment variable. If the section name appears within the value of that environment variable, then the returned function operates similar to console.error(). If not, then the returned function is a no-op.

import { debuglog } from 'node:util';
const log = debuglog('foo');

log('hello from foo [%d]', 123);

If this program is run with NODE_DEBUG=foo in the environment, then it will output something like:

FOO 3245: hello from foo [123]

where 3245 is the process id. If it is not run with that environment variable set, then it will not print anything.

The section supports wildcard also:

import { debuglog } from 'node:util';
const log = debuglog('foo-bar');

log('hi there, it\'s foo-bar [%d]', 2333);

if it is run with NODE_DEBUG=foo* in the environment, then it will output something like:

FOO-BAR 3257: hi there, it's foo-bar [2333]

Multiple comma-separated section names may be specified in the NODE_DEBUG environment variable: NODE_DEBUG=fs,net,tls.

The optional callback argument can be used to replace the logging function with a different function that doesn't have any initialization or unnecessary wrapping.

import { debuglog } from 'node:util';
let log = debuglog('internals', (debug) => {
// Replace with a logging function that optimizes out
// testing if the section is enabled
log = debug;
});

A string identifying the portion of the application for which the debuglog function is being created.

A callback invoked the first time the logging function is called with a function argument that is a more optimized logging function.

The logging function

util.diff() compares two string or array values and returns an array of difference entries. It uses the Myers diff algorithm to compute minimal differences, which is the same algorithm used internally by assertion error messages.

If the values are equal, an empty array is returned.

const { diff } = require('node:util');

// Comparing strings
const actualString = '12345678';
const expectedString = '12!!5!7!';
console.log(diff(actualString, expectedString));
// [
// [0, '1'],
// [0, '2'],
// [1, '3'],
// [1, '4'],
// [-1, '!'],
// [-1, '!'],
// [0, '5'],
// [1, '6'],
// [-1, '!'],
// [0, '7'],
// [1, '8'],
// [-1, '!'],
// ]
// Comparing arrays
const actualArray = ['1', '2', '3'];
const expectedArray = ['1', '3', '4'];
console.log(diff(actualArray, expectedArray));
// [
// [0, '1'],
// [1, '2'],
// [0, '3'],
// [-1, '4'],
// ]
// Equal values return empty array
console.log(diff('same', 'same'));
// []

The first value to compare

The second value to compare

An array of difference entries. Each entry is an array with two elements:

The util.format() method returns a formatted string using the first argument as a printf-like format string which can contain zero or more format specifiers. Each specifier is replaced with the converted value from the corresponding argument. Supported specifiers are:

If a specifier does not have a corresponding argument, it is not replaced:

util.format('%s:%s', 'foo');
// Returns: 'foo:%s'

Values that are not part of the format string are formatted using util.inspect() if their type is not string.

If there are more arguments passed to the util.format() method than the number of specifiers, the extra arguments are concatenated to the returned string, separated by spaces:

util.format('%s:%s', 'foo', 'bar', 'baz');
// Returns: 'foo:bar baz'

If the first argument does not contain a valid format specifier, util.format() returns a string that is the concatenation of all arguments separated by spaces:

util.format(1, 2, 3);
// Returns: '1 2 3'

If only one argument is passed to util.format(), it is returned as it is without any formatting:

util.format('%% %s');
// Returns: '%% %s'

util.format() is a synchronous method that is intended as a debugging tool. Some input values can have a significant performance overhead that can block the event loop. Use this function with care and never in a hot code path.

A printf-like format string.

This function is identical to format, except in that it takes an inspectOptions argument which specifies options that are passed along to inspect.

util.formatWithOptions({ colors: true }, 'See object %O', { foo: 42 });
// Returns 'See object { foo: 42 }', where `42` is colored as a number
// when printed to a terminal.

Returns an array of call site objects containing the stack of the caller function.

Unlike accessing an error.stack, the result returned from this API is not interfered with Error.prepareStackTrace.

import { getCallSites } from 'node:util';

function exampleFunction() {
const callSites = getCallSites();

console.log('Call Sites:');
callSites.forEach((callSite, index) => {
console.log(`CallSite ${index + 1}:`);
console.log(`Function Name: ${callSite.functionName}`);
console.log(`Script Name: ${callSite.scriptName}`);
console.log(`Line Number: ${callSite.lineNumber}`);
console.log(`Column Number: ${callSite.columnNumber}`);
});
// CallSite 1:
// Function Name: exampleFunction
// Script Name: /home/example.js
// Line Number: 5
// Column Number: 26

// CallSite 2:
// Function Name: anotherFunction
// Script Name: /home/example.js
// Line Number: 22
// Column Number: 3

// ...
}

// A function to simulate another stack layer
function anotherFunction() {
exampleFunction();
}

anotherFunction();

It is possible to reconstruct the original locations by setting the option sourceMap to true. If the source map is not available, the original location will be the same as the current location. When the --enable-source-maps flag is enabled, for example when using --experimental-transform-types, sourceMap will be true by default.

import { getCallSites } from 'node:util';

interface Foo {
foo: string;
}

const callSites = getCallSites({ sourceMap: true });

// With sourceMap:
// Function Name: ''
// Script Name: example.js
// Line Number: 7
// Column Number: 26

// Without sourceMap:
// Function Name: ''
// Script Name: example.js
// Line Number: 2
// Column Number: 26

Number of frames to capture as call site objects. Default: 10. Allowable range is between 1 and 200.

An array of call site objects

Returns an array of call site objects containing the stack of the caller function.

Unlike accessing an error.stack, the result returned from this API is not interfered with Error.prepareStackTrace.

import { getCallSites } from 'node:util';

function exampleFunction() {
const callSites = getCallSites();

console.log('Call Sites:');
callSites.forEach((callSite, index) => {
console.log(`CallSite ${index + 1}:`);
console.log(`Function Name: ${callSite.functionName}`);
console.log(`Script Name: ${callSite.scriptName}`);
console.log(`Line Number: ${callSite.lineNumber}`);
console.log(`Column Number: ${callSite.columnNumber}`);
});
// CallSite 1:
// Function Name: exampleFunction
// Script Name: /home/example.js
// Line Number: 5
// Column Number: 26

// CallSite 2:
// Function Name: anotherFunction
// Script Name: /home/example.js
// Line Number: 22
// Column Number: 3

// ...
}

// A function to simulate another stack layer
function anotherFunction() {
exampleFunction();
}

anotherFunction();

It is possible to reconstruct the original locations by setting the option sourceMap to true. If the source map is not available, the original location will be the same as the current location. When the --enable-source-maps flag is enabled, for example when using --experimental-transform-types, sourceMap will be true by default.

import { getCallSites } from 'node:util';

interface Foo {
foo: string;
}

const callSites = getCallSites({ sourceMap: true });

// With sourceMap:
// Function Name: ''
// Script Name: example.js
// Line Number: 7
// Column Number: 26

// Without sourceMap:
// Function Name: ''
// Script Name: example.js
// Line Number: 2
// Column Number: 26

An array of call site objects

Returns a Map of all system error codes available from the Node.js API. The mapping between error codes and error names is platform-dependent. See Common System Errors for the names of common errors.

fs.access('file/that/does/not/exist', (err) => {
const errorMap = util.getSystemErrorMap();
const name = errorMap.get(err.errno);
console.error(name); // ENOENT
});

Returns the string message for a numeric error code that comes from a Node.js API. The mapping between error codes and string messages is platform-dependent.

fs.access('file/that/does/not/exist', (err) => {
const message = util.getSystemErrorMessage(err.errno);
console.error(message); // no such file or directory
});

Returns the string name for a numeric error code that comes from a Node.js API. The mapping between error codes and error names is platform-dependent. See Common System Errors for the names of common errors.

fs.access('file/that/does/not/exist', (err) => {
const name = util.getSystemErrorName(err.errno);
console.error(name); // ENOENT
});

Usage of util.inherits() is discouraged. Please use the ES6 class and extends keywords to get language level inheritance support. Also note that the two styles are semantically incompatible.

Inherit the prototype methods from one constructor into another. The prototype of constructor will be set to a new object created from superConstructor.

This mainly adds some input validation on top of Object.setPrototypeOf(constructor.prototype, superConstructor.prototype). As an additional convenience, superConstructor will be accessible through the constructor.super_ property.

const util = require('node:util');
const EventEmitter = require('node:events');

function MyStream() {
EventEmitter.call(this);
}

util.inherits(MyStream, EventEmitter);

MyStream.prototype.write = function(data) {
this.emit('data', data);
};

const stream = new MyStream();

console.log(stream instanceof EventEmitter); // true
console.log(MyStream.super_ === EventEmitter); // true

stream.on('data', (data) => {
console.log(`Received data: "${data}"`);
});
stream.write('It works!'); // Received data: "It works!"

ES6 example using class and extends:

import EventEmitter from 'node:events';

class MyStream extends EventEmitter {
write(data) {
this.emit('data', data);
}
}

const stream = new MyStream();

stream.on('data', (data) => {
console.log(`Received data: "${data}"`);
});
stream.write('With ES6');

The util.inspect() method returns a string representation of object that is intended for debugging. The output of util.inspect may change at any time and should not be depended upon programmatically. Additional options may be passed that alter the result. util.inspect() will use the constructor's name and/or Symbol.toStringTag property to make an identifiable tag for an inspected value.

class Foo {
get [Symbol.toStringTag]() {
return 'bar';
}
}

class Bar {}

const baz = Object.create(null, { [Symbol.toStringTag]: { value: 'foo' } });

util.inspect(new Foo()); // 'Foo [bar] {}'
util.inspect(new Bar()); // 'Bar {}'
util.inspect(baz); // '[foo] {}'

Circular references point to their anchor by using a reference index:

import { inspect } from 'node:util';

const obj = {};
obj.a = [obj];
obj.b = {};
obj.b.inner = obj.b;
obj.b.obj = obj;

console.log(inspect(obj));
// {
// a: [ [Circular *1] ],
// b: { inner: [Circular *2], obj: [Circular *1] }
// }

The following example inspects all properties of the util object:

import util from 'node:util';

console.log(util.inspect(util, { showHidden: true, depth: null }));

The following example highlights the effect of the compact option:

import { inspect } from 'node:util';

const o = {
a: [1, 2, [[
'Lorem ipsum dolor sit amet,\nconsectetur adipiscing elit, sed do ' +
'eiusmod \ntempor incididunt ut labore et dolore magna aliqua.',
'test',
'foo']], 4],
b: new Map([['za', 1], ['zb', 'test']]),
};
console.log(inspect(o, { compact: true, depth: 5, breakLength: 80 }));

// { a:
// [ 1,
// 2,
// [ [ 'Lorem ipsum dolor sit amet,\nconsectetur [...]', // A long line
// 'test',
// 'foo' ] ],
// 4 ],
// b: Map(2) { 'za' => 1, 'zb' => 'test' } }

// Setting `compact` to false or an integer creates more reader friendly output.
console.log(inspect(o, { compact: false, depth: 5, breakLength: 80 }));

// {
// a: [
// 1,
// 2,
// [
// [
// 'Lorem ipsum dolor sit amet,\n' +
// 'consectetur adipiscing elit, sed do eiusmod \n' +
// 'tempor incididunt ut labore et dolore magna aliqua.',
// 'test',
// 'foo'
// ]
// ],
// 4
// ],
// b: Map(2) {
// 'za' => 1,
// 'zb' => 'test'
// }
// }

// Setting `breakLength` to e.g. 150 will print the "Lorem ipsum" text in a
// single line.

The showHidden option allows WeakMap and WeakSet entries to be inspected. If there are more entries than maxArrayLength, there is no guarantee which entries are displayed. That means retrieving the same WeakSet entries twice may result in different output. Furthermore, entries with no remaining strong references may be garbage collected at any time.

import { inspect } from 'node:util';

const obj = { a: 1 };
const obj2 = { b: 2 };
const weakSet = new WeakSet([obj, obj2]);

console.log(inspect(weakSet, { showHidden: true }));
// WeakSet { { a: 1 }, { b: 2 } }

The sorted option ensures that an object's property insertion order does not impact the result of util.inspect().

import { inspect } from 'node:util';
import assert from 'node:assert';

const o1 = {
b: [2, 3, 1],
a: '`a` comes before `b`',
c: new Set([2, 3, 1]),
};
console.log(inspect(o1, { sorted: true }));
// { a: '`a` comes before `b`', b: [ 2, 3, 1 ], c: Set(3) { 1, 2, 3 } }
console.log(inspect(o1, { sorted: (a, b) => b.localeCompare(a) }));
// { c: Set(3) { 3, 2, 1 }, b: [ 2, 3, 1 ], a: '`a` comes before `b`' }

const o2 = {
c: new Set([2, 1, 3]),
a: '`a` comes before `b`',
b: [2, 3, 1],
};
assert.strict.equal(
inspect(o1, { sorted: true }),
inspect(o2, { sorted: true }),
);

The numericSeparator option adds an underscore every three digits to all numbers.

import { inspect } from 'node:util';

const thousand = 1000;
const million = 1000000;
const bigNumber = 123456789n;
const bigDecimal = 1234.12345;

console.log(inspect(thousand, { numericSeparator: true }));
// 1_000
console.log(inspect(million, { numericSeparator: true }));
// 1_000_000
console.log(inspect(bigNumber, { numericSeparator: true }));
// 123_456_789n
console.log(inspect(bigDecimal, { numericSeparator: true }));
// 1_234.123_45

util.inspect() is a synchronous method intended for debugging. Its maximum output length is approximately 128 MiB. Inputs that result in longer output will be truncated.

Any JavaScript primitive or Object.

The representation of object.

The util.inspect() method returns a string representation of object that is intended for debugging. The output of util.inspect may change at any time and should not be depended upon programmatically. Additional options may be passed that alter the result. util.inspect() will use the constructor's name and/or Symbol.toStringTag property to make an identifiable tag for an inspected value.

class Foo {
get [Symbol.toStringTag]() {
return 'bar';
}
}

class Bar {}

const baz = Object.create(null, { [Symbol.toStringTag]: { value: 'foo' } });

util.inspect(new Foo()); // 'Foo [bar] {}'
util.inspect(new Bar()); // 'Bar {}'
util.inspect(baz); // '[foo] {}'

Circular references point to their anchor by using a reference index:

import { inspect } from 'node:util';

const obj = {};
obj.a = [obj];
obj.b = {};
obj.b.inner = obj.b;
obj.b.obj = obj;

console.log(inspect(obj));
// {
// a: [ [Circular *1] ],
// b: { inner: [Circular *2], obj: [Circular *1] }
// }

The following example inspects all properties of the util object:

import util from 'node:util';

console.log(util.inspect(util, { showHidden: true, depth: null }));

The following example highlights the effect of the compact option:

import { inspect } from 'node:util';

const o = {
a: [1, 2, [[
'Lorem ipsum dolor sit amet,\nconsectetur adipiscing elit, sed do ' +
'eiusmod \ntempor incididunt ut labore et dolore magna aliqua.',
'test',
'foo']], 4],
b: new Map([['za', 1], ['zb', 'test']]),
};
console.log(inspect(o, { compact: true, depth: 5, breakLength: 80 }));

// { a:
// [ 1,
// 2,
// [ [ 'Lorem ipsum dolor sit amet,\nconsectetur [...]', // A long line
// 'test',
// 'foo' ] ],
// 4 ],
// b: Map(2) { 'za' => 1, 'zb' => 'test' } }

// Setting `compact` to false or an integer creates more reader friendly output.
console.log(inspect(o, { compact: false, depth: 5, breakLength: 80 }));

// {
// a: [
// 1,
// 2,
// [
// [
// 'Lorem ipsum dolor sit amet,\n' +
// 'consectetur adipiscing elit, sed do eiusmod \n' +
// 'tempor incididunt ut labore et dolore magna aliqua.',
// 'test',
// 'foo'
// ]
// ],
// 4
// ],
// b: Map(2) {
// 'za' => 1,
// 'zb' => 'test'
// }
// }

// Setting `breakLength` to e.g. 150 will print the "Lorem ipsum" text in a
// single line.

The showHidden option allows WeakMap and WeakSet entries to be inspected. If there are more entries than maxArrayLength, there is no guarantee which entries are displayed. That means retrieving the same WeakSet entries twice may result in different output. Furthermore, entries with no remaining strong references may be garbage collected at any time.

import { inspect } from 'node:util';

const obj = { a: 1 };
const obj2 = { b: 2 };
const weakSet = new WeakSet([obj, obj2]);

console.log(inspect(weakSet, { showHidden: true }));
// WeakSet { { a: 1 }, { b: 2 } }

The sorted option ensures that an object's property insertion order does not impact the result of util.inspect().

import { inspect } from 'node:util';
import assert from 'node:assert';

const o1 = {
b: [2, 3, 1],
a: '`a` comes before `b`',
c: new Set([2, 3, 1]),
};
console.log(inspect(o1, { sorted: true }));
// { a: '`a` comes before `b`', b: [ 2, 3, 1 ], c: Set(3) { 1, 2, 3 } }
console.log(inspect(o1, { sorted: (a, b) => b.localeCompare(a) }));
// { c: Set(3) { 3, 2, 1 }, b: [ 2, 3, 1 ], a: '`a` comes before `b`' }

const o2 = {
c: new Set([2, 1, 3]),
a: '`a` comes before `b`',
b: [2, 3, 1],
};
assert.strict.equal(
inspect(o1, { sorted: true }),
inspect(o2, { sorted: true }),
);

The numericSeparator option adds an underscore every three digits to all numbers.

import { inspect } from 'node:util';

const thousand = 1000;
const million = 1000000;
const bigNumber = 123456789n;
const bigDecimal = 1234.12345;

console.log(inspect(thousand, { numericSeparator: true }));
// 1_000
console.log(inspect(million, { numericSeparator: true }));
// 1_000_000
console.log(inspect(bigNumber, { numericSeparator: true }));
// 123_456_789n
console.log(inspect(bigDecimal, { numericSeparator: true }));
// 1_234.123_45

util.inspect() is a synchronous method intended for debugging. Its maximum output length is approximately 128 MiB. Inputs that result in longer output will be truncated.

Any JavaScript primitive or Object.

The representation of object.

Returns true if there is deep strict equality between val1 and val2. Otherwise, returns false.

See assert.deepStrictEqual() for more information about deep strict equality.

Provides a higher level API for command-line argument parsing than interacting with process.argv directly. Takes a specification for the expected arguments and returns a structured object with the parsed options and positionals.

import { parseArgs } from 'node:util';
const args = ['-f', '--bar', 'b'];
const options = {
foo: {
type: 'boolean',
short: 'f',
},
bar: {
type: 'string',
},
};
const {
values,
positionals,
} = parseArgs({ args, options });
console.log(values, positionals);
// Prints: [Object: null prototype] { foo: true, bar: 'b' } []

Used to provide arguments for parsing and to configure the parser. config supports the following properties:

The parsed command line arguments:

Stability: 1.1 - Active development Given an example .env file:

import { parseEnv } from 'node:util';

parseEnv('HELLO=world\nHELLO=oh my\n');
// Returns: { HELLO: 'oh my' }

The raw contents of a .env file.

Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);
promisifiedStat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});

Or, equivalently using async functions:

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);

async function callStat() {
const stats = await promisifiedStat('.');
console.log(`This directory is owned by ${stats.uid}`);
}

callStat();

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:

import { promisify } from 'node:util';

class Foo {
constructor() {
this.a = 42;
}

bar(callback) {
callback(null, this.a);
}
}

const foo = new Foo();

const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'

Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);
promisifiedStat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});

Or, equivalently using async functions:

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);

async function callStat() {
const stats = await promisifiedStat('.');
console.log(`This directory is owned by ${stats.uid}`);
}

callStat();

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:

import { promisify } from 'node:util';

class Foo {
constructor() {
this.a = 42;
}

bar(callback) {
callback(null, this.a);
}
}

const foo = new Foo();

const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'

Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);
promisifiedStat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});

Or, equivalently using async functions:

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);

async function callStat() {
const stats = await promisifiedStat('.');
console.log(`This directory is owned by ${stats.uid}`);
}

callStat();

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:

import { promisify } from 'node:util';

class Foo {
constructor() {
this.a = 42;
}

bar(callback) {
callback(null, this.a);
}
}

const foo = new Foo();

const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'

Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);
promisifiedStat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});

Or, equivalently using async functions:

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);

async function callStat() {
const stats = await promisifiedStat('.');
console.log(`This directory is owned by ${stats.uid}`);
}

callStat();

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:

import { promisify } from 'node:util';

class Foo {
constructor() {
this.a = 42;
}

bar(callback) {
callback(null, this.a);
}
}

const foo = new Foo();

const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'

Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);
promisifiedStat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});

Or, equivalently using async functions:

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);

async function callStat() {
const stats = await promisifiedStat('.');
console.log(`This directory is owned by ${stats.uid}`);
}

callStat();

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:

import { promisify } from 'node:util';

class Foo {
constructor() {
this.a = 42;
}

bar(callback) {
callback(null, this.a);
}
}

const foo = new Foo();

const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'

Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);
promisifiedStat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});

Or, equivalently using async functions:

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);

async function callStat() {
const stats = await promisifiedStat('.');
console.log(`This directory is owned by ${stats.uid}`);
}

callStat();

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:

import { promisify } from 'node:util';

class Foo {
constructor() {
this.a = 42;
}

bar(callback) {
callback(null, this.a);
}
}

const foo = new Foo();

const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'

Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);
promisifiedStat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});

Or, equivalently using async functions:

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);

async function callStat() {
const stats = await promisifiedStat('.');
console.log(`This directory is owned by ${stats.uid}`);
}

callStat();

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:

import { promisify } from 'node:util';

class Foo {
constructor() {
this.a = 42;
}

bar(callback) {
callback(null, this.a);
}
}

const foo = new Foo();

const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'

Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);
promisifiedStat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});

Or, equivalently using async functions:

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);

async function callStat() {
const stats = await promisifiedStat('.');
console.log(`This directory is owned by ${stats.uid}`);
}

callStat();

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:

import { promisify } from 'node:util';

class Foo {
constructor() {
this.a = 42;
}

bar(callback) {
callback(null, this.a);
}
}

const foo = new Foo();

const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'

Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);
promisifiedStat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});

Or, equivalently using async functions:

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);

async function callStat() {
const stats = await promisifiedStat('.');
console.log(`This directory is owned by ${stats.uid}`);
}

callStat();

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:

import { promisify } from 'node:util';

class Foo {
constructor() {
this.a = 42;
}

bar(callback) {
callback(null, this.a);
}
}

const foo = new Foo();

const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'

Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);
promisifiedStat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});

Or, equivalently using async functions:

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);

async function callStat() {
const stats = await promisifiedStat('.');
console.log(`This directory is owned by ${stats.uid}`);
}

callStat();

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:

import { promisify } from 'node:util';

class Foo {
constructor() {
this.a = 42;
}

bar(callback) {
callback(null, this.a);
}
}

const foo = new Foo();

const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'

Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);
promisifiedStat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});

Or, equivalently using async functions:

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);

async function callStat() {
const stats = await promisifiedStat('.');
console.log(`This directory is owned by ${stats.uid}`);
}

callStat();

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:

import { promisify } from 'node:util';

class Foo {
constructor() {
this.a = 42;
}

bar(callback) {
callback(null, this.a);
}
}

const foo = new Foo();

const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'

Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);
promisifiedStat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});

Or, equivalently using async functions:

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);

async function callStat() {
const stats = await promisifiedStat('.');
console.log(`This directory is owned by ${stats.uid}`);
}

callStat();

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:

import { promisify } from 'node:util';

class Foo {
constructor() {
this.a = 42;
}

bar(callback) {
callback(null, this.a);
}
}

const foo = new Foo();

const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'

Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);
promisifiedStat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});

Or, equivalently using async functions:

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);

async function callStat() {
const stats = await promisifiedStat('.');
console.log(`This directory is owned by ${stats.uid}`);
}

callStat();

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:

import { promisify } from 'node:util';

class Foo {
constructor() {
this.a = 42;
}

bar(callback) {
callback(null, this.a);
}
}

const foo = new Foo();

const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'

Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);
promisifiedStat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});

Or, equivalently using async functions:

import { promisify } from 'node:util';
import { stat } from 'node:fs';

const promisifiedStat = promisify(stat);

async function callStat() {
const stats = await promisifiedStat('.');
console.log(`This directory is owned by ${stats.uid}`);
}

callStat();

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:

import { promisify } from 'node:util';

class Foo {
constructor() {
this.a = 42;
}

bar(callback) {
callback(null, this.a);
}
}

const foo = new Foo();

const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'

That can be used to declare custom promisified variants of functions.

Enable or disable printing a stack trace on SIGINT. The API is only available on the main thread.

Returns str with any ANSI escape codes removed.

console.log(util.stripVTControlCharacters('\u001B[4mvalue\u001B[0m'));
// Prints "value"

This function returns a formatted text considering the format passed for printing in a terminal. It is aware of the terminal's capabilities and acts according to the configuration set via NO_COLOR, NODE_DISABLE_COLORS and FORCE_COLOR environment variables.

import { styleText } from 'node:util';
import { stderr } from 'node:process';

const successMessage = styleText('green', 'Success!');
console.log(successMessage);

const errorMessage = styleText(
'red',
'Error! Error!',
// Validate if process.stderr has TTY
{ stream: stderr },
);
console.error(errorMessage);

util.inspect.colors also provides text formats such as italic, and underline and you can combine both:

console.log(
util.styleText(['underline', 'italic'], 'My italic underlined message'),
);

When passing an array of formats, the order of the format applied is left to right so the following style might overwrite the previous one.

console.log(
util.styleText(['red', 'green'], 'text'), // green
);

The special format value none applies no additional styling to the text.

The full list of formats can be found in modifiers.

A text format or an Array of text formats defined in util.inspect.colors.

The text to to be formatted.

Returns the string after replacing any surrogate code points (or equivalently, any unpaired surrogate code units) with the Unicode "replacement character" U+FFFD.

Creates and returns an AbortController instance whose AbortSignal is marked as transferable and can be used with structuredClone() or postMessage().

A transferable AbortController

Marks the given AbortSignal as transferable so that it can be used withstructuredClone() and postMessage().

const signal = transferableAbortSignal(AbortSignal.timeout(100));
const channel = new MessageChannel();
channel.port2.postMessage(signal, [signal]);

The AbortSignal

The same AbortSignal

Returns the 1-based column offset on the line for the associated function call.

Returns the name of the function associated with this call site.

Returns the number, 1-based, of the line for the associate function call.

Returns the unique id of the script, as in Chrome DevTools protocol Runtime.ScriptId.

Returns the name of the resource that contains the script for the function for this call site.

Returns the name of the function. Function names are read-only and can not be changed.

Determines whether the given value inherits from this function if this function was used as a constructor function.

A constructor function can control which objects are recognized as its instances by 'instanceof' by overriding this method.

Calls the function, substituting the specified object for the this value of the function, and the specified array for the arguments of the function.

The object to be used as the this object.

A set of arguments to be passed to the function.

For a given function, creates a bound function that has the same body as the original function. The this object of the bound function is associated with the specified object, and has the specified initial parameters.

An object to which the this keyword can refer inside the new function.

A list of arguments to be passed to the new function.

Calls a method of an object, substituting another object for the current object.

The object to be used as the current object.

A list of arguments to be passed to the method.

Returns a string representation of a function.

Returns the name of the function. Function names are read-only and can not be changed.

Determines whether the given value inherits from this function if this function was used as a constructor function.

A constructor function can control which objects are recognized as its instances by 'instanceof' by overriding this method.

Calls the function, substituting the specified object for the this value of the function, and the specified array for the arguments of the function.

The object to be used as the this object.

A set of arguments to be passed to the function.

For a given function, creates a bound function that has the same body as the original function. The this object of the bound function is associated with the specified object, and has the specified initial parameters.

An object to which the this keyword can refer inside the new function.

A list of arguments to be passed to the new function.

Calls a method of an object, substituting another object for the current object.

The object to be used as the current object.

A list of arguments to be passed to the method.

Returns a string representation of a function.

The util.debuglog().enabled getter is used to create a test that can be used in conditionals based on the existence of the NODE_DEBUG environment variable. If the section name appears within the value of that environment variable, then the returned value will be true. If not, then the returned value will be false.

import { debuglog } from 'node:util';
const enabled = debuglog('foo').enabled;
if (enabled) {
console.log('hello from foo [%d]', 123);
}

If this program is run with NODE_DEBUG=foo in the environment, then it will output something like:

hello from foo [123]

Reconstruct the original location in the stacktrace from the source-map. Enabled by default with the flag --enable-source-maps.

The length at which input values are split across multiple lines. Set to Infinity to format the input as a single line (in combination with compact set to true or any number >= 1).

If true, the output is styled with ANSI color codes. Colors are customizable.

Setting this to false causes each object key to be displayed on a new line. It will also add new lines to text that is longer than breakLength. If set to a number, the most n inner elements are united on a single line as long as all properties fit into breakLength. Short array elements are also grouped together. Note that no text will be reduced below 16 characters, no matter the breakLength size. For more information, see the example below.

If false, [util.inspect.custom](depth, opts, inspect) functions are not invoked.

Specifies the number of times to recurse while formatting object. This is useful for inspecting large objects. To recurse up to the maximum call stack size pass Infinity or null.

If set to true, getters are going to be inspected as well. If set to 'get' only getters without setter are going to be inspected. If set to 'set' only getters having a corresponding setter are going to be inspected. This might cause side effects depending on the getter function.

Specifies the maximum number of Array, TypedArray, WeakMap, and WeakSet elements to include when formatting. Set to null or Infinity to show all elements. Set to 0 or negative to show no elements.

Specifies the maximum number of characters to include when formatting. Set to null or Infinity to show all elements. Set to 0 or negative to show no characters.

If set to true, an underscore is used to separate every three digits in all bigints and numbers.

If true, object's non-enumerable symbols and properties are included in the formatted result. WeakMap and WeakSet entries are also included as well as user defined prototype properties (excluding method properties).

If true, Proxy inspection includes the target and handler objects.

If set to true or a function, all properties of an object, and Set and Map entries are sorted in the resulting string. If set to true the default sort is used. If set to a function, it is used as a compare function.

The length at which input values are split across multiple lines. Set to Infinity to format the input as a single line (in combination with compact set to true or any number >= 1).

If true, the output is styled with ANSI color codes. Colors are customizable.

Setting this to false causes each object key to be displayed on a new line. It will also add new lines to text that is longer than breakLength. If set to a number, the most n inner elements are united on a single line as long as all properties fit into breakLength. Short array elements are also grouped together. Note that no text will be reduced below 16 characters, no matter the breakLength size. For more information, see the example below.

If false, [util.inspect.custom](depth, opts, inspect) functions are not invoked.

Specifies the number of times to recurse while formatting object. This is useful for inspecting large objects. To recurse up to the maximum call stack size pass Infinity or null.

If set to true, getters are going to be inspected as well. If set to 'get' only getters without setter are going to be inspected. If set to 'set' only getters having a corresponding setter are going to be inspected. This might cause side effects depending on the getter function.

Specifies the maximum number of Array, TypedArray, WeakMap, and WeakSet elements to include when formatting. Set to null or Infinity to show all elements. Set to 0 or negative to show no elements.

Specifies the maximum number of characters to include when formatting. Set to null or Infinity to show all elements. Set to 0 or negative to show no characters.

If set to true, an underscore is used to separate every three digits in all bigints and numbers.

If true, object's non-enumerable symbols and properties are included in the formatted result. WeakMap and WeakSet entries are also included as well as user defined prototype properties (excluding method properties).

If true, Proxy inspection includes the target and handler objects.

If set to true or a function, all properties of an object, and Set and Map entries are sorted in the resulting string. If set to true the default sort is used. If set to a function, it is used as a compare function.

If true, prototype and constructor comparison is skipped during deep strict equality check.

If true, allows explicitly setting boolean options to false by prefixing the option name with --no-.

Whether this command accepts positional arguments.

Array of argument strings.

Used to describe arguments known to the parser.

Should an error be thrown when unknown arguments are encountered, or when arguments are passed that do not match the type configured in options.

Return the parsed tokens. This is useful for extending the built-in behavior, from adding additional checks through to reprocessing the tokens in different ways.

The value to assign to the option if it does not appear in the arguments to be parsed. The value must match the type specified by the type property. If multiple is true, it must be an array. No default value is applied when the option does appear in the arguments to be parsed, even if the provided value is falsy.

Whether this option can be provided multiple times. If true, all values will be collected in an array. If false, values for the option are last-wins.

A single character alias for the option.

Type of argument.

A stream that will be validated if it can be colored.

When true, stream is checked to see if it can handle colors.

Type of argument used in parseArgs.