Front end JS for generating image thumbnails from base64-encoded images.

ThumbnailGenerator.js

function ThumbnailGenerator() {
  this.resizeCanvas = document.createElement('canvas');

  this.generate = function (imgSrc, thumbDims, compression) {
    [this.resizeCanvas.width, this.resizeCanvas.height] = [thumbDims.x, thumbDims.y];
    const ctx = this.resizeCanvas.getContext("2d");
  
    const tmp = new Image();
  
    const ret = new Promise(resolve => {
      tmp.onload = () => {
        ctx.drawImage(tmp, 0, 0, thumbDims.x, thumbDims.y);
        resolve(this.resizeCanvas.toDataURL('image/jpeg', compression || 0.5));
      };
    });
    tmp.src = imgSrc;
    return ret;
  };

  this.generateBatch = function (imgSrcs, thumbDims, compression) {
    return Promise.all(imgSrcs.map(img => this.generate(img, thumbDims, compression)));
  }

  return this;
}