Promises in Node.js: then, catch, and chaining

Promises Represent Future Values

A Promise is an object that represents an asynchronous operation that will complete (or fail) in the future. Instead of passing a callback into a function, the function returns a Promise you can chain handlers onto.

const fs = require('fs').promises;

fs.readFile('./data.txt', 'utf8')
  .then(data => {
    console.log(data);
    return fs.writeFile('./copy.txt', data);
  })
  .then(() => console.log('Copy written'))
  .catch(err => console.error('Error:', err.message))
  .finally(() => console.log('Done'));

Promisify Callback APIs

Use util.promisify to wrap any error-first callback function into a Promise-returning one:

const { promisify } = require('util');
const readFile = promisify(require('fs').readFile);

readFile('./data.txt', 'utf8')
  .then(data => console.log(data))
  .catch(err => console.error(err));

Promise.all for Parallel Operations

Run multiple async operations concurrently and wait for all to complete:

const [a, b] = await Promise.all([
  fs.readFile('a.txt', 'utf8'),
  fs.readFile('b.txt', 'utf8')
]);

Use Promise.allSettled when you want results even if some fail — it never rejects.