(Hopefully) generates a truly random ID-String. It should have an entropy of 6 * length bits. Browser support: IE11+, any other sane browser. ( http://caniuse.com/#search=crypto )

generateRandomID.js

// my usage:
// var userId = generateRandomID(16) // 16 * 6 bits = 96 bits of entropy.  

function generateRandomID(length) {
    var cryptoObject = (window.crypto || window["msCrypto"]);
    if (!cryptoObject || !cryptoObject.getRandomValues) {
        throw "No supported cryptography-handler found!";
    }
    var base64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_";
    var array = new Uint8Array(length);
    cryptoObject.getRandomValues(array);
    var out = "";
    for (var i = 0; i < array.length; i++) {
        out += base64[array[i] & 63];
    }
    return out;
}