mapAsync.js

var noop = function(){ };

function map(list, func, callback) {
  var result = new Array(list.length);
  var count = 0;
  list.forEach(iterator);

  function iterator (item, index) {
    func(item, executed); 

    function executed(err, ret) {
      if (err) {
       callback(err);
       callback = noop;
        return;
      }

      result[index] = ret;
      count++;
      if (count === list.length)
        return  callback(null, result);
    } 
  }
}

module.exports = map;

mapSync.js

function map(list, func, callback) {
  var result = [];

  func(list.shift(), cb);
  
  function cb (err, ret) {
    if (err) {
      callback(err, null);
      return;
    }

    result.push(ret);
    
    if (!list.length) {
      callback(null, result);
      return;
    }

    func(list.shift(), cb);
  }
}

module.exports = map;

test.js

var assert = require('assert');
var map    = require('./map');
describe('Map', function(){
  it('should return all values multiplied by 2', function(done) {

    var arr = [1, 2, 3, 4, 4, 5, 10, -1, 20000, 9090];

    function func (a, cb) {
      var delay = (Math.random()*10).toFixed(0)%3;
      setTimeout(function () {
        cb(2*a);
      }, delay*1000);
    }

    function callback (err, result) {
      if (err) {
        console.log(err);
      } else {
        assert.equal(
          result.toString(),
          [2, 4, 6, 8, 8, 10, 20, -2, 40000, 18180].toString()
        );
        done();
      }
    }
    map(arr, func, callback);
  });
});