Asynchronously append data to a file in Node.js.

append_file.js

import * as fs from 'fs';
import * as fsPromises from 'fs/promises';

const file = 'hello_world.txt';
const data = 'Hello, World!';

//
// Callback API
//

fs.appendFile(file, data, 'utf8', error => {
  if (error) {
    throw error;
  } else {
    console.log(`Appended string "${data}" to ${file}.`);
  }
});


//
// Promises API
//

// fsPromises
//   .appendFile(file, data, 'utf8')
//   .then(() => console.log(`Appended string "${data}" to ${file}.`))
//   .catch(console.error);

package.json

{
  "name": "append_file",
  "version": "1.0.0",
  "description": "Asynchronously append data to a file.",
  "private": true,
  "main": "append_file.js",
  "type": "module",
  "scripts": {
    "start": "node append_file.js",
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "Kieran Barker <kieran@barker.codes> (https://barker.codes/)",
  "license": "MIT"
}

Read the blog post.