JSFiddle - React, Tailwind, and code Playground

JavaScript

function preloadImages(urls, allImagesLoadedCallback){
	var loadedCounter = 0;
  var toBeLoadedNumber = urls.length;
  urls.forEach(function(url){
  	preloadImage(url, function(){
    	loadedCounter++;
			console.log('Number of loaded images: ' + loadedCounter);
      if(loadedCounter == toBeLoadedNumber){
      	allImagesLoadedCallback();
      }
    });
  });
  function preloadImage(url, anImageLoadedCallback){
      var img = new Image();
      img.src = url;
      img.onload = anImageLoadedCallback;
  }
}

preloadImages([
	'//upload.wikimedia.org/wikipedia/commons/d/da/Internet2.jpg',
  '//www.csee.umbc.edu/wp-content/uploads/2011/08/www.jpg'
], function(){
	console.log('All images were loaded');
});