hulu-medium-preload.js

componentWillMount(){
    this.preloadImg();
  }

...

preloadImg(){
  const imgURLs = imageURLsFromData;

  for(let i = 0; i<imgURLs.length; i++){
    let downloadingImg = new Image();
    downloadingImg.src = imgURLs[i];
    downloadingImg.onload = () =>{
        imgLoadCount++;
        if(imgLoadCount === imgURLs.length){
          //initially set to "in progress"
          this.setState({loading:"done"});
       }
     }
  } 
}