setImmediate multiple implementations

set-immediate.js

var setImmediate = (function(f, q, p, reg) {
    q = [];
    f = document.createElement('iframe');
    f.style.cssText = 'position:absolute;top:-999em;left:0;width:1px;height:1px;';
    document.body.appendChild(f);
    function done() {
        for (var i=0; i<q.length; i++) q[i]();
        q.length = 0;
    }
    p = [
        function(fn){
            var w = f.contentWindow;
            w.onmessage = fn;
            w.postMessage('', '*');
        },
        function(fn){
            var img = new Image();
            img.onerror = fn;
            img.src = '';
        },
        function(fn){ setTimeout(fn, 0); }
    ];
    reg = function pick() {
        p.forEach(function(o) {
            try {
                o(function() {
                    if (reg===pick) reg=o, done();
                });
            }catch(e){}
        });
    };
    return function(fn) {
        if (q.push(fn)===1) reg(done);
    };
}());