setImmediate polyfill using Promise

setImmediate.js

// Note: You probably do not want to use this in production code, as Promise is
//   not supported by all browsers yet.

(function() {
    "use strict";

    if (window.setImmediate) {
        return;
    }

    var pending = {},
        nextHandle = 1;

    function onResolve(handle) {
        var callback = pending[handle];
        if (callback) {
            delete pending[handle];
            callback.fn.apply(null, callback.args);
        }
    }

    window.setImmediate = function(fn) {
        var args = Array.prototype.slice.call(arguments, 1),
            handle;

        if (typeof fn !== "function") {
            throw new TypeError("invalid function");
        }

        handle = nextHandle++;
        pending[handle] = { fn: fn, args: args };

        new Promise(function(resolve) {
            resolve(handle);
        }).then(onResolve);
        
        return handle;
    };

    window.clearImmediate = function(handle) {
        delete pending[handle];
    };
}());