Promisification of an async code in JavaScript

Since promises have been around in JavaScript not for a long time, some asynchronous functionality may not support promises. In this case, it is useful to create a promise wrapper over such a code, since it is much more convenient to use promises. Such a transformation is called promisification.

An example of functionality that doesn't support promises is an image loading, which we have already discussed in previous lessons:

javascriptlet image = document.createElement('img');
image.src = 'img.png';

image.addEventListener('load', function() {
  document.body.appendChild(image);
});
image.addEventListener('error', function() {
  console.log('image load error');
});

Let's implement a promisification of this code by wrapping it in a function that returns the promise:

javascriptfunction loadImage(path) {
  return new Promise(function(resolve, reject) {
    let image = document.createElement('img');
    image.src = path;
    
    image.addEventListener('load', function() {
      resolve(image);
    });
    image.addEventListener('error', function() {
      reject(new Error('image "' + path + '"  load error'));
    });
  });
}

We can use our function like this:

javascriptloadImage('img.png').then(function(image) {
  document.body.appendChild(image);
}).catch(function(error) {
  console.log(error);
});

javascriptlet paths = ['img1.png', 'img2.png', 'img3.png'];

javascriptwindow.addEventListener('DOMContentLoaded', function() {
  console.log('dom loaded');
});