Simple asynchronous mapper in javascript

gistfile1.js

Array.prototype.mapAsync = function(mapper, finalcallback) {
  var waiting = 0;
  var that = this;
  var newlist = [];
  this.forEach(function(item) {
    // console.log('starting async mapper', item);
    waiting ++;
    setTimeout(function() {
      mapper(item, function(newitem) {
        newlist.push(newitem);
        // console.log('async mapper done', item);
        setTimeout(function() {
          waiting --;
          if (waiting === 0) {
            // console.log('firing final callback');
            finalcallback(newlist);
          }
        }, 1);
      });
    }, 1);
  });
};