A setImmediate polyfill optimized for size.

setImmediate.js

(function(global, name, mc, counter, slice){
  if (!(name in global)) {
    if (mc = mc in global && new global[mc]) {
      mc.port1.onmessage = function(evt) {
        slice[evt = evt.data]();
        delete slice[evt];
      };

      global[name] = function(cb) {
        var args = slice.call(arguments, 1);
        slice[++counter] = function() { cb.apply(0, args); };
        mc.port2.postMessage(counter);
      }
    }
  }
})(this, 'setImmediate', 'MessageChannel', 0, [].slice);