A basic implementation of promises in javascript, covers Promise/A+ specification

promise.js

function Promise(callback) {  
  let state = 'pending';
  let value;
  let deferred = null;

  const resolve = function(newValue) {
    if(newValue && typeof newValue.then === 'function') {
      newValue.then(resolve, reject);
      return;
    }
    state = 'resolved';
    value = newValue;

    if(deferred)
      handle(deferred);
  }

  const reject = function(reason) {
    state = 'rejected';
    value = reason;

    if(deferred)
      handle(deferred);
  }

  function handle(handler) {
    if(state === 'pending') {
      deferred = handler;
      return;
    }

    const handlerCallback;

    if(state === 'resolved') {
      handlerCallback = handler.onResolved;
    else
      handlerCallback = handler.onRejected;

    if(!handlerCallback) {
      if(state === 'resolved')
        handler.resolve(value);
      else
        handler.reject(value);
      
      return;
    }

    const ret = handlerCallback(value);
    handler.resolve(ret);
  }

  this.then = function(onResolved, onRejected) {
    return new Promise(function(resolve, reject) {
      handle({
        onResolved: onResolved,
        onRejected: onRejected,
        resolve: resolve,
        reject: reject
      });
    });
  };

  callback(resolve, reject);
}