How to generate system unique UUID

GetUUID.js

function uuid()
{
    var char = '0123456789abcdef'.split('');
    
    var uuid = [], rnd = Math.random, r;
    uuid[8] = uuid[13] = uuid[18] = uuid[23] = '-';
    uuid[14] = '4'; // version 4
    
    for (var i = 0; i < 36; i++)
    {
        if (!uuid[i])
        {
            r = 0 | rnd()*16;
            
            uuid[i] = char[(i == 19) ? (r & 0x3) | 0x8 : r & 0xf];
        }
    }
    
    return uuid.join('');
}