Asynchronous map #nodejs

amap.js

// amap -- asynchronous map
//
// Maps fn over list and passes the result to callback.  For example,
//
//   function loadEntries(folder, callback) {
//     fs.readdir(folder, function(err, files) {
//       if (err) throw err;
//       amap(files, callback, function(name, index, next) {
//         load(path.join(folder, name), next);
//       });
//     });
//   }

function amap(list, callback, fn) {
  var result = [],
      index = 0,
      limit = list.length;

  function next(err, value) {
    if (err)
      callback(err);
    else {
      result.push(value);
      if (++index < limit) {
        fn(list[index], index, next);
      }
      else
        callback(null, result);
    }
  }

  return limit ? fn(list[0], 0, next) : callback([]);
}