gistfile1.txt

let img = new Image();
img.addEventListener('load', function(){
    var canvas = document.createElement("canvas");
    canvas.width = this.width;
    canvas.height = this.height;

    // Copy the image contents to the canvas
    var ctx = canvas.getContext("2d");
    ctx.drawImage(this, 0, 0);

    // Get the data-URL formatted image
    // Firefox supports PNG and JPEG. You could check img.src to
    // guess the original format, but be aware the using "image/jpg"
    // will re-encode the image.
    var dataURL = canvas.toDataURL("image/png");

    let data = dataURL.replace(/^data:image\/(png|jpg);base64,/, "");

});
img.src = "...";