gistfile1.js

<script>
    reader.onloadend = function (e) {
        var myImage = new Image(); // Creates image object
        myImage.src = e.target.result; // Assigns converted image to image object
        myImage.onload = function(ev) {
           var myCanvas = document.getElementById("myCanvas"); // Creates a canvas object
           var myContext = myCanvas.getContext("2d"); // Creates a contect object
           myCanvas.width = myImage.width; // Assigns image's width to canvas
           myCanvas.height = myImage.height; // Assigns image's height to canvas
           myContext.drawImage(myImage,0,0); // Draws the image on canvas
           let imgData = myCanvas.toDataURL("image/jpeg",0.75); // Assigns image base64 string in jpeg format to a variable
        }
    }
</script>