Node fs.appendFile function | API Reference | Bun

Asynchronously append data to a file, creating the file if it does not yet exist. data can be a string or a Buffer.

The mode option only affects the newly created file. See open for more details.

import { appendFile } from 'node:fs';

appendFile('message.txt', 'data to append', (err) => {
if (err) throw err;
console.log('The "data to append" was appended to file!');
});

If options is a string, then it specifies the encoding:

import { appendFile } from 'node:fs';

appendFile('message.txt', 'data to append', 'utf8', callback);

The path may be specified as a numeric file descriptor that has been opened for appending (using fs.open() or fs.openSync()). The file descriptor will not be closed automatically.

import { open, close, appendFile } from 'node:fs';

function closeFd(fd) {
close(fd, (err) => {
if (err) throw err;
});
}

open('message.txt', 'a', (err, fd) => {
if (err) throw err;

try {
appendFile(fd, 'data to append', 'utf8', (err) => {
closeFd(fd);
if (err) throw err;
});
} catch (err) {
closeFd(fd);
throw err;
}
});

filename or file descriptor

Asynchronously append data to a file, creating the file if it does not exist.

A path to a file. If a URL is provided, it must use the file: protocol. If a file descriptor is provided, the underlying file will not be closed automatically.

The data to write. If something other than a Buffer or Uint8Array is provided, the value is coerced to a string.