Javascript blob to base64

helpers.js

/** Get a base64 string representing the content of the provided Blob
 *
 * @param blob
 * The blob object
 *
 * @param cb
 * A callback called with the base64 string on success
 *
 * @return undefined
 *
 * @example
 * blob_to_base64(input_file_id.files[0], b64string => { console.log(b64string); });
 */
function blob_to_base64(blob, cb) {
    const reader = new FileReader();
    reader.readAsDataURL(blob); 
    reader.onloadend = function() {
        const base64data = reader.result;
        /* Remove the base64 header added by the Javascript API. */
        const actual_data = base64data.slice(
            (base64data.indexOf(";base64")+";base64".length+1));
        cb(actual_data);
    }
}