Promises are an alternative to callbacks for delivering the results of an asynchronous computation. First, let's look at the basic structure of promises, before we briefly look at the advantages of using promises over normal callbacks.

Let's rewrite the code above with promises:

const fs = require('fs');
const readFile = (file) => 
{
  return new Promise((resolve, reject) => 
  {
    fs.readFile(file, 'utf8', (err, data) => 
    {
      if (err) reject(err);
      else resolve(data);
    });
  });
}
// call the async function
readFile(`${__dirname}/../temp/sample.txt`)
  .then(data => console.log(data))
  .catch(error => console.log('err: ', error.message));

This code can further be simplified by using the util.promisify function, which takes a function following the common Node.js callback style, that is, taking an (err, value) => … callback as the last argument and returning a version that returns promises:

const fs = require('fs');
const util = require('util');
const readFile = util.promisify(fs.readFile);
readFile(`${__dirname}/../temp/sample.txt`, 'utf8')
  .then(data => console.log(data))
  .catch(error => console.log('err: ', error));

From what we have seen so far, promises provide a standard way of handling asynchronous code, making it a little more readable.

What if you had 10 files, and you wanted to read all of them? Promise.all comes to the rescue. Promise.all is a handy function that enables you to run asynchronous functions in parallel. Its input is an array of promises; its output is a single promise that is fulfilled with an array of the results:

const fs = require('fs');
const util = require('util');
const readFile = util.promisify(fs.readFile);
const files = [
  'temp/sample.txt',
  'temp/sample1.txt',
  'temp/sample2.txt',
];
// map the files to the readFile function, creating an
// array of promises
const promises = files.map(file => readFile(`${__dirname}/../${file}`, 'utf8'));
Promise.all(promises)
  .then(data => 
  {
    data.forEach(text => console.log(text));
  })
  .catch(error => console.log('err: ', error));