Read image data

function set_canvas_image(f, element) {
    let reader = new FileReader();

    // Closure to capture the file information.
    reader.onload = (function() {
    return function(e) {
        // Render thumbnail.
        element.src = e.target.result
    };
    })(f);

    // Read in the image file as a data URL.
    reader.readAsDataURL(f);

}