CODE EXAMPLE FOR JAVASCRIPT

function resizeBase64Img(base64, newWidth, newHeight) { return new Promise((resolve, reject)=>{

/**
 * Resize a base 64 Image
 * @param {String} base64 - The base64 string (must include MIME type)
 * @param {Number} newWidth - The width of the image in pixels
 * @param {Number} newHeight - The height of the image in pixels
 */
function resizeBase64Img(base64, newWidth, newHeight) {
    return new Promise((resolve, reject)=>{
        var canvas = document.createElement("canvas");
        canvas.style.width = newWidth.toString()+"px";
        canvas.style.height = newHeight.toString()+"px";
        let context = canvas.getContext("2d");
        let img = document.createElement("img");
        img.src = base64;
        img.onload = function () {
            context.scale(newWidth/img.width,  newHeight/img.height);
            context.drawImage(img, 0, 0); 
            resolve(canvas.toDataURL());               
        }
    });
}