load images in parallel

load-images.js

const loadImage = (url, callback) => {
  const image = new Image();
  image.onload = () => {
    callback(null, image);
  };
  image.onerror = () => {
    callback(new Error('Could not load image at ' + url));
  };
  image.src = url;
}

function loadImages(urls, callback) {
  var returned = false;
  var count = 0;
  var result = new Array(urls.length);
  urls.forEach((url, index) => {
    loadImage(url, (error, item) => {
      if (returned) return;
      if (error) {
        returned = true;
        return callback(error);
      }
      result[index] = item;
      count++;
      if (count === urls.length) {
        return callback(null, result);
      }
    });
  });
}

// const imageUrls = ['one.png', 'two.png', 'three.png'];
// loadImages(imageUrls, function(err, images) {
//   if (err) throw err;
//   console.log('All images loaded', images);
// });