Preload images in plain JavaScript

preload.js

function preloadImages(srcs, onComplete = f => f) {
  var imgArray = {};
  var imgErrorArray = {};

  var imageFailCount = 0;
  var imageLoadedCount = 0;

  srcs.forEach(function(src, index) {
    imgArray[index] = null;
    imgErrorArray[index] = null;

    var img = new Image();
    img.src = src;

    img.onload = function() {
      imgArray[index] = img;
      imageLoadedCount++;

      if (imageLoadedCount + imageFailCount === srcs.length) {
        onComplete(imgArray, !!imageFailCount, imgErrorArray);
      }
    };

    img.onerror = function(err) {
      imageFailCount++;
      imgErrorArray[index] = err;

      if (imageLoadedCount + imageFailCount === srcs.length) {
        onComplete(imgArray, !!imageFailCount, imgErrorArray);
      }
    };
  });
}

var srcs = [
  './assets/img1.jpg',
  './assets/img2.jpg',
  './assets/img3.jpg',
  './assets/img4.jpg',
  './assets/img5.jpg',
  './assets/img6.jpg',
  './assets/img7.jpg',
  './assets/img8.jpg',
  './assets/img10.jpg',
];

preloadImages(srcs, function(images, err, errDetails) {
  console.log('Completed', images, err, errDetails);
});