Node util.promisify function | API Reference | Bun

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.