Determine if string is in base64 using JavaScript

If you want to check whether it can be decoded or not, you can simply try decoding it and see whether it failed:

try {
    window.atob(str);
} catch(e) {
    // something failed

    // if you want to be specific and only catch the error which means
    // the base 64 was invalid, then check for 'e.code === 5'.
    // (because 'DOMException.INVALID_CHARACTER_ERR === 5')
}