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A static class in javascript that can preload images using promises in a async manner so your main thread would not hang or slow down.

as lots of us already know that javascript is a single threaded language but in javascript you can do multiple operation in a single thread async.

async meaning you can run multiple operation in parallel without disturbing main thread  which is mainly UI thread in javascript.


promises is one way to do operation in async manner 

learn more about promises



below is my solution to preload images in a async manner in javascript




//preloading images using javascript
class Loader
{
    constructor()
    {
      this.progressCallBack=undefined;
    }
    //load all images in aysnc manner
      //load multiple images
   static async loadImages(args) {
    //a promiseArray that can hold all Promises 
    let promiseStack = [];
       
    //total images u want to load
    const totalImages = args.length;

    //number of images loading
    let loadedImages = 0;
    for (let i = 0; i < totalImages; i++) {
     
      promiseStack.push(
        new Promise((accept, reject) => {
         const image = new Image();
          image.src = args[i];
          //on each image load progess how many images you loaded yet
          image.onload = evt => {
            loadedImages++;
            const per = Number((loadedImages * 100) / totalImages);
            if (this.progressCallBack) {
              this.progressCallBack(per);
            }
            accept(evt);
          };
          //reject the promise a console back a message when a image failed to load
          image.onerror=evt=>{
            console.warn(`this image is not loaded=>${evt.target.src}`);
          }
        })
      );
    }
    

    //reslove all promise all send send back boolean you can check that all images loaded successfully
    return new Promise((accept, recject) => {
      Promise.all(promiseStack)
        .then(r => {
          accept(true);
        })
        .catch(Error => {
          reject("some images missed")
        });
    });
  }
}
//now load images in a async manner 
  const images=["https://picsum.photos/id/237/200/32",
      "https://picsum.photos/seed/picsum/200/300",
        "https://picsum.photos/200/300?grayscale",
        "https://picsum.photos/200/300/?blur",
        "https://ragingbanana.files.wordpress.com/2015/03/35f7d4fa3fdf5ae46be107004f93497a.png",
"https://www.spottedzebra.us/img/posts/2013-05-20-XAML%20Spritesheet%20Animation/scared-owl-spritesheet.png"
      ]
      Loader.loadImages(images).then((r)=>{
       // console.log(r);
      })
      Loader.progressCallBack=(p)=>{
          this.p=p;
           // console.log(this.p);
      }


    


working link -->


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