Quickly returns a random UUID in JavaScript that is compliant with RFC-4122 Version 4.

uuid.js

/**
 * Quickly returns a random UUID that is compliant with RFC-4122 Version 4.
 *
 * An example result is "2c3fa383-0d9d-4104-8fa2-58fdf614f021"
 *
 * Works in modern browsers and IE11+, and not Android Browser. A fallback if
 * you care about those is to drop the Uint32Array and crypto lines, and
 * replace the dX = buf[X] lines with dX = Math.random() * 0x100000000 >>> 0.
 * In node.js do the same but use dX = crypto.randomBytes(4).readUInt32BE(0).
 *
 * This is a modified version of this StackOverflow answer by Jeff Ward:
 * http://stackoverflow.com/a/21963136/843621
 *
 * @license MIT license
 */
var uuid = (function() {
    var lut = [], // look-up table to convert decimals (0-256) to hexadecimals (0x00-0xff)
        buf = new Uint32Array(4); // random value buffer
    for (var i = 0; i < 256; i++) {
        lut[i] = (i < 16 ? '0' : '') + (i).toString(16);
    }
    return function() {
        window.crypto.getRandomValues(buf);
        var d0 = buf[0],
            d1 = buf[1],
            d2 = buf[2],
            d3 = buf[3];
        return lut[d0&0xff]+     lut[d0>>8&0xff]+    lut[d0>>16&0xff]+     lut[d0>>24&0xff]+'-'+
               lut[d1&0xff]+     lut[d1>>8&0xff]+'-'+lut[d1>>16&0x0f|0x40]+lut[d1>>24&0xff]+'-'+
               lut[d2&0x3f|0x80]+lut[d2>>8&0xff]+'-'+lut[d2>>16&0xff]+     lut[d2>>24&0xff]+
               lut[d3&0xff]     +lut[d3>>8&0xff]+    lut[d3>>16&0xff]+     lut[d3>>24&0xff];
    };
})();