Generate a UUID v4 in JavaScript

/**
 * UUID (Universally Unique Identifier) version 4 generator.
 *
 * @return {string} UUID (Universally Unique Identifier) version 4 generator.
 */
function uuid4() {
    var isUndefined = function (value) {
        return typeof value === "undefined";
    };

    var crypto = window.crypto || window.msCrypto;

    if (!isUndefined(crypto) && crypto.getRandomValues) {
        // Use window.crypto API if available
        var arr = new Uint16Array(8);
        crypto.getRandomValues(arr);

        // set 4 in byte 7
        arr[3] = (arr[3] & 0xfff) | 0x4000;

        // set 2 most significant bits of byte 9 to '10'
        arr[4] = (arr[4] & 0x3fff) | 0x8000;

        var pad = function (num) {
            var v = num.toString(16);
            while (v.length < 4) {
                v = "0" + v;
            }
            return v;
        };

        return (
            pad(arr[0]) +
            pad(arr[1]) +
            pad(arr[2]) +
            pad(arr[3]) +
            pad(arr[4]) +
            pad(arr[5]) +
            pad(arr[6]) +
            pad(arr[7])
        );
    } else {
        // http://stackoverflow.com/questions/105034/how-to-create-a-guid-uuid-in-javascript/2117523#2117523
        return "xxxxxxxxxxxx4xxxyxxxxxxxxxxxxxxx".replace(/[xy]/g, function (
            c
        ) {
            var r = (Math.random() * 16) | 0,
                v = c === "x" ? r : (r & 0x3) | 0x8;
            return v.toString(16);
        });
    }
}