Generating a GUID for javascript applications and storing it in HTML5 local strorage.

guid.js

function generate_guid() {
    return 'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'.replace(/[xy]/g, function(c) {
        var r = Math.random()*16|0, v = c == 'x' ? r : (r&0x3|0x8);
        return v.toString(16);
    });
}

function get_guid() {
    if (window.localStorage.guid != null) {
        return window.localStorage.guid;
    } else {
        window.localStorage.guid = generate_guid();
        return window.localStorage.guid;
    }
}