Bringing addListeners and removeListeners to Promises.

Promise.listeners.js

(function (wm) {'use strict';
  // original proposal via Oliver Dunk @oliverdunk_ via ES ML
  // (c) Andrea Giammarchi - (ISC)
  Object.defineProperties(
    Promise.prototype,
    {
      addListeners: {
        configurable: true,
        value: function addListeners(resolve, reject) {
          var once = wm.get(this);
          if (!once) {
            wm.set(this, once = this.then(
              function (result) {
                once.result = result;
                each(result, once.resolve, once.reject);
              },
              function (error) {
                once.result = error;
                each(error, once.reject, once.resolve);
              }
            ));
            once.resolve = [];
            once.reject = [];
          }
          if (once.hasOwnProperty('result')) {
            this.then(resolve || Object, reject || Object);
          } else {
            if (resolve && once.resolve.indexOf(resolve) < 0)
              once.resolve.push(resolve);
            if (reject && once.reject.indexOf(reject) < 0)
              once.reject.push(reject);
          }
          return this;
        }
      },
      removeListeners: {
        configurable: true,
        value: function removeListeners(resolve, reject) {
          var i, once = wm.get(this);
          if (once) {
            if (resolve) {
              i = once.resolve.indexOf(resolve);
              if (-1 < i) once.resolve.splice(i, 1);
            }
            if (reject) {
              i = once.reject.indexOf(reject);
              if (-1 < i) once.reject.splice(i, 1);
            }
          }
          return this;
        }
      }
    }
  );
  function each(info, toInvoke, toClean) {
    toClean.splice(0);
    toInvoke.splice(0).forEach(invoke, info);
  }
  function invoke(fn) {
    fn(this);
  }
}(new WeakMap));

Test via:

var promise = new Promise(res => setTimeout(res, 1000, 'OK'))
  .addListeners(console.log, console.error);

// after 1 second
promise.addListeners(console.log, console.error);

or via

new Promise(res => setTimeout(res, 1000, 'OK'))
  .addListeners(console.log, console.error)
  .removeListeners(console.log);

and fail via

new Promise((res, rej)=> setTimeout(rej, 1000, 'Fail'))
  .addListeners(console.log, console.error);