Asynchronous mapper in javascript with timeout

gistfile1.js

Array.prototype.mapAsync = function(mapper, finalcallback, timeout) {
  var mappertimeout = timeout || 10000;
  var waiting = 0;
  var that = this;
  var newlist = [];
  var _donecallback = function() {
    waiting --;
    console.log('after async mapper done');
    if (waiting === 0) {
      console.log('firing final callback');
      finalcallback(newlist);
    }
  };
  this.forEach(function(item) {
    console.log('starting async mapper', item, waiting);
    waiting ++;
    setTimeout(function() {
      var imdone = false;
      setTimeout(function() {
        if (imdone) return;
        console.log('mapper timed out!', item);
        imdone = true;
        _donecallback();
      }, mappertimeout);
      try {
        mapper(item, function(newitem) {
          if (imdone) return;
          console.log('mapper done.', item, newitem);
          imdone = true;
          if (typeof newitem !== 'undefined') newlist.push(newitem);
          _donecallback();
        });
      } catch(error) {
        console.error('mapper crashed', item, error);
        if (imdone) return;
        imdone = true;
        _donecallback();
      }
    }, 1);
  });
};