Simple re-implementation of Dojo's Deferred constructor

Deferred.js

function Deferred() {
  this.chain = [];
}
Deferred.prototype.addBoth = function (cb) {
  this.chain.push([cb, cb]);
  return this;
}
Deferred.prototype.addCallback = function (cb) {
  this.chain.push([cb, null]);
  return this;
}
Deferred.prototype.addCallbacks = function (cb, eb) {
  this.chain.push([cb, eb]);
  return this;
}
Deferred.prototype.addErrback = function (eb) {
  this.chain.push([null, eb]);
  return this;
}
Deferred.prototype.callback = function (res) {
  this.chain.forEach(function (pair) {
    if (pair[0] !== null) {
      res = pair[0](res);
    }
  });
  return res;
}
Deferred.prototype.errback = function (res) {
  this.chain.forEach(function (pair) {
    if (pair[1] !== null) {
      res = pair[1](res);
    }
  });
  return res;
}