如何用node.js生成唯一的ID

function generate(count) {    var founded = false,        _sym = 'abcdefghijklmnopqrstuvwxyz1234567890',        str = '';    while(!founded) {        for(var i = 0; i < count; i++) {            str += _sym[parseInt(Math.random() * (_sym.length))];        }        base.getID(string, function(err, res) {            if(!res.length) {                founded = true; // How to do it?            }        });    }    return str;}function generate(count) {    var founded = fal