UUID version 4 using browser crypto API. Not particularly optimised.

uuid.v4.js

function makeUuid(){
    var randomValues = window.crypto.getRandomValues(new window.Uint8ClampedArray(16));
    var randomHex = '';

    for (var i = 0; i < randomValues.length - 1; i++) {
        var hex = randomValues[i].toString(16);

        if (hex.length === 0) {
            randomHex += '00';
        } else if (hex.length === 1) {
            randomHex += '0' + hex;
        } else {
            randomHex += hex;
        }
    }

    var special;

    if (randomValues[15] < 64) {
        special = '8';
    } else if (randomValues[15] < 128) {
        special = '9';
    } else if (randomValues[15] < 192) {
        special = 'A';
    } else {
        special = 'B';
    }

    return [
        randomHex.slice(0, 8),
        randomHex.slice(8, 12),
        '4' + randomHex.slice(12, 15),
        special + randomHex.slice(15, 18),
        randomHex.slice(18, 30)
    ].join('-');
}