Promise A+ Implementation

promise.js

"use strict"
var Promise = function () {
  this.state = 'pending'
  this.thenables = []
}

Promise.prototype.resolve = function (value) {
  if (this.state != 'pending') return

  this.state = 'fulfilled'
  this.value = value
  this._handleThen()
  return this
}

Promise.prototype.reject = function (reason) {
  if (this.state != 'pending') return

  this.state = 'rejected'
  this.reason = reason
  this._handleThen()
  return this
}

Promise.prototype._handleThen = function () {
  if (this.state == 'pending') return

  if (this.thenables.length) {
    for (var i = 0; i < this.thenables.length; i++) {
      var thenPromise = this.thenables[i].promise
      var returnedVal

      try {
        switch (this.state) {
          case 'fulfilled':
            if (this.thenables[i].fulfill) {
              returnedVal = this.thenables[i].fulfill(this.value)
            } else {
              thenPromise.resolve(this.value)
            }
            break
          case 'rejected':
            if (this.thenables[i].reject) {
              returnedVal = this.thenables[i].reject(this.reason)
            } else {
              thenPromise.reject(this.reason)
            }
            break
        }

          if (returnedVal == null) { // ???
            this.thenables[i].promise.resolve(returnedVal)
          }
          else if (returnedVal instanceof Promise || typeof returnedVal.then == 'function') {
            returnedVal.then(thenPromise.resolve.bind(thenPromise), thenPromise.reject.bind(thenPromise))
          }
          else {
            this.thenables[i].promise.resolve(returnedVal)
          }
      } catch (e) {
        thenPromise.reject(e)
      }
    }
    this.thenables = []
  }
}

Promise.prototype.then = function (onFulfilled, onRejected) {
  var thenable = {}

  if (typeof onFulfilled == 'function') {
    thenable.fulfill = onFulfilled
  }

  if (typeof onRejected == 'function') {
    thenable.reject = onRejected
  }

  if (this.state != 'pending') {
    setImmediate(function () {
      this._handleThen()
    }.bind(this))
  }

  thenable.promise = new Promise()
  this.thenables.push(thenable)

  return thenable.promise
}

module.exports = Promise

test-adapter.js

"use strict"
var promisesTest = require('promises-aplus-tests')
var Promise = require('./promise')

var adapter = {
  fulfilled: function (value) {
    var promise = new Promise()
    return promise.resolve(value)
  },
  rejected: function (reason) {
    var promise = new Promise()
    return promise.reject(reason)
  },
  pending: function () {
    var promise = new Promise()
    return {
      promise: promise,
      fulfill: promise.resolve.bind(promise),
      reject: promise.reject.bind(promise)
    }
  }
}

promisesTest(adapter, function (er) {
  if (er) console.error(er)
  process.exit()
})