JavaScript uuid

uuid.js

function uuid4() {
    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);
        });
    }
}