Prophecy – Angular 2.0 Deferred/Promise

Status: In-Development

This project makes available an ES6 Deferred implementation, using ES6 Promises. Also included is a utility mock implementation of Promise with a corresponding PromiseBackend which allows flushing of the Promise's underlying microtask queue, allowing developers to write synchronous tests against Promise-based libraries.

Install

$ npm install --save angular/prophecy

Deferred

The Deferred class is a small wrapper around Promise which lifts the requirement of resolving the promise within the resolver function that gets passed to the Promise constructor.

Example with vanilla Promise:

var promise = new Promise(function(resolve, reject) {
  setTimeout(function() {
    resolve('done');
  }, 1000);
});
promise.then(function(val) {
  console.log(val);
});
//Logs "done" to the console in 1 second

Example with Deferred without using PromiseMock:

import {Deferred} from './node_modules/prophecy/src/Deferred';
var deferred = new Deferred();
deferred.promise.then(function(val) {
  console.log(val);
});
setTimeout(function() {
  deferred.resolve('done');
}, 1000);
//Logs "done" to the console in 1 secondn

Deferred Instance Methods and Properties

name	description
resolve(value)	Alias to promise.resolve()
reject(reason)	Alias to promise.reject()
promise	Promise instance, used to chain actions to be executed upon fulfillment of the promise

PromiseMock Module (For Testing)

The PromiseMock module contains two classes that allow synchronous testing of promise-based APIs. The PromiseBackend class provides methods to register and unregister the PromiseMock class as the global Promise constructor, as well as methods to flush the queue of pending operations registered by the PromiseMock. The PromiseMock implementation is exactly the same as the native/Traceur ES6 Promise, except that it adds its pending tasks to the flushable PromiseBackend queue instead of a hidden microtask queue.

Example test of Deferred using PromiseBackend:

import {PromiseBackend} from './node_modules/prophecy/src/PromiseMock';
import {Deferred} from './node_modules/prophecy/src/Deferred';
describe('.resolve()', function() {
  it('should call the resolver\'s resolve function with the correct value',
    function() {
      var resolveSpy = jasmine.createSpy('resolveSpy');
      PromiseBackend.forkZone().run(function() {
        var deferred = new Deferred();
        deferred.promise.then(resolveSpy);
        deferred.resolve('Flush me!');
        PromiseBackend.flush(true);
      });

      expect(resolveSpy).toHaveBeenCalledWith('Flush me!');
  });
});

PromiseBackend

The PromiseBackend class is completely static. This class manages the process of patching the global object with PromiseMock as well as flushing any pending promise fulfillment operations. The PromiseBackend keeps a single queue of pending tasks, which is shared by all promises.

The PromiseBackend provides a convenience method to create a zone within which tests can be executed, which will automatically patch and unpatch window.Promise. The zone will also verify that no outstanding requests are waiting to be flushed.

beforeEach(function() {
  //No need for PromiseBackend.patchWithMock(), or an afterEach() to unpatch
  this.zone = PromiseBackend.forkZone();
});
it('should resolve with a smiley', function() {
  this.zone.run(function() {
    var resolveSpy = jasmine.createSpy();
    var backend = new PromiseBackend();
    new Promise(function(resolve) {
      resolve(':)');
    }).
    then(resolveSpy);
    backend.flush();
    expect(resolveSpy).toHaveBeenCalledWith(':)');
  });
})

The flush method is called in lieu of waiting for the next VM turn, and prevents the need for writing async tests using setTimeout. Example writing an test that waits for an async operation with setTimeout:

it('should resolve with a smiley', function(done) {
  var resolveSpy = jasmine.createSpy();
  new Promise(function(resolve) {
    resolve(':)');
  }).
  then(resolveSpy);
  setTimeout(function() {
    expect(resolveSpy).toHaveBeenCalledWith(':)');
    done();
  }, 0);
});

With PromiseMock and PromiseBackend.flush(), this same test can be expressed as:

it('should resolve with a smiley', function() {
  var resolveSpy = jasmine.createSpy();
  PromiseBackend.forkZone().run(function() {
    new Promise(function(resolve) {
      resolve(':)');
    }).
    then(resolveSpy);
    PromiseBackend.flush(true);
  });

  expect(resolveSpy).toHaveBeenCalledWith(':)');
});

PromiseBackend Methods and Properties

name	description
static setGlobal(global:Object)	global context to which the native implementation of Promise is attached (default: window)
static flush(recursiveFlush=false)	Flushes all tasks that have been queued for execution. If recursiveFlush is true, the backend will continue flushing until the queue is empty, including tasks that have been added since flushing began. Returns the PromiseBackend instance to allow chaining.
static executeAsap(fn:Function)	Add a function to the queue to be executed on the next flush
static restoreNativePromise()	Restore the native Promise implementation to the global object
static patchWithMock()	Replace the global Promise constructor with PromiseMock
static verifyNoOutstandingTasks()	Throw if tasks are in the queue waiting to flush
static zone:forkZone()	Creates and returns a new zone which automatically patches window.Promise with the PromiseMock before execution, and restores the original promise after execution.
static queue:Array.<Function>	Array of functions to be executed on next flush, populated by executeAsap(). Note that this is undefined until an instance of PromiseBackend is created.
global:Object	The global context within which PromiseBackend is operating, default: window

Design Doc (superceded by implementation in this project).

TODO

• Add A+ tests for PromiseMock. This implementation is copied from Traceur (which is ported from V8). The Traceur implementation is already passing A+ tests. This project should have the tests as well.
• Add src/index.js to export items that should be available at runtime.
• Add build process