Calculate base64 string size in bytes

calculateBase64Size.js

var calculateBase64Size = function (base64, mimeType){
    var b64Length = base64.length;
    var base64Padding = 0;

    if (base64.substring(b64Length - 1) === '=') base64Padding = 1;
    else if (base64.substring(b64Length - 2) === '==') base64Padding = 2;
    b64Length = base64.length - ('data:' + mimeType + ';base64,').length;
    return (b64Length / 4) * 3 - base64Padding;
};