Mock Promises

Mock Promises is a library for synchronously testing asynchronous javascript promises. It is designed to feel similar to libraries for synchronously testing asynchronous http requests, such as jasmine-ajax.

Supported Libraries

Mock Promises currently supports es6-promise, bluebird, Q and native Promises (where available). If you would like to use Mock Promises for a library that is not supported, please open a github issue.

Mock Promises is test framework agnostic, and we have code examples for the jasmine and mocha testing frameworks in the spec/javascripts directory.

Installation

Self Vendoring

Download mock-promises.js and add it to your project. If you are using the jasmine gem, mock-promises.js needs to be in the src_files path in jasmine.yml.

NPM

In node, you can use npm install mock-promises.

Once the module is installed require('mock-promises') in your specs will attach mockPromises to the global namespace.

Node does not currently support native promises and you may need to use es6-promise if you want to run the example specs.

Promise Libraries

Setup

To start mocking, use the install function. The argument to install is the Promise class used by your promise library.

• Q

mockPromises.install(Q.makePromise)

• ES6-Promise

mockPromises.install(ES6Promise.Promise)

• Other Libraries

In principal, mock promises can be used with any testing library that mostly uses then under the hood. It does rely a bit on internal state, so each library probably needs a small amount of work to use (or a large amount for native promises).

It is recommended to put the install is in the global beforeEach of your spec helper. Any promises that are instantiated before you start mocking will not be mocked.

To prevent test pollution, you should reset mocking between tests

mockPromises.reset();

Teardown

To turn off mocking, use the uninstall function

mockPromises.uninstall();

Native Promises

If you are using Native promises, mock promises needs to mock out the constructor, which requires getMockPromise. This method is doing a lot more than install and may have slightly different behavior than mocking out promises from libraries.

Promise = mockPromises.getMockPromise(Promise);

to turn off mock in this case, there is a getOriginalPromise method

Promise = mockPromises.getOriginalPromise();

Promise Resolution Policy

Promises often lead to other promises, for example, promise.then(function1).then(function2), so we had to decide what happens to the function2 on a promise when you execute the function1. We have chosen to encourage the user to explicitly ask for each callback to be executed, so that they do not accidentally execute callacks without realizing it. Even tick will only go down one level of the chain unless the user specifies the number of levels. We have recently provided tickAllTheWay, but its use is discouraged in most circumstances.

API

install(PromiseClass)

Starts mocking promises of the given Promise Class

uninstall()

Stops mocking promises mocked by install

reset()

Resets all Contracts.

getMockPromise(PromiseClass)

Returns a mocked version of PromiseClass; needed for mocking native promises

getOriginalPromise

Returns the unmocked version of PromiseClass mocked by getMockPromise; needed for unmocking native promises

tick(count)

Executes all fulfillmentHandlers for each resolved promise. Executes all rejection handlers for each rejected promise. If passed a count, tick will repeat this procedure that many times. This is useful for deeply chained thens.

tickAllTheWay()

Repeats the tick procedure until there are no more resolved or rejected promises with unexectued handlers. This method is discouraged when testing code that you control. Using tick by itself with a specific count leads to much better understanding of your code flow and fewer potential race conditions.

executeForPromise(mockedPromise)

Executes all fulfillmentHandlers if the mocked promise is resolved. Executes all rejectionHandlers if the mocked promise is rejected. Will not execute handlers that have already been executed.

executeForPromises(mockedPromises)

Calls executeForPromise on each mocked promise in the array of mocked promises, in order.

iterateForPromise(mockedPromise)

In the event of a tree of promises created by chaining then off of mockedPromise, this will go down the tree and find the first level that has not yet been exectued and then execute it. If top-level callbacks on mockedPromise have not been executed, this has the same effect as executeForPromise. If the entire tree has already been executed, nothing happens.

iterateForPromises(mockedPromises)

Calls iterateForProimse on each mocked promise in the array of mocked promisees, in order.

valueForPromise(mockedPromise)

Returns the resolved value of the mocked promise without executing any of its callbacks.

Contracts

Everytime then is called on a mocked promise, it generates a contract within mock promises. A contract represents a promise and a set of handlers. These are mostly used internally, but can be useful for debugging purposes.

contracts.all()

Returns an array of all available contracts.

contracts.forPromise(mockedPromise)

Returns an array of all contracts associated with the mocked promise.

Examples

To see more detailed examples, look in mock-promises_spec.js. Some examples are included below.

describe("executeForPromise", function() {
  var promise1, promise2;
  beforeEach(function() {
    mockPromises.install(Q.makePromise);
    mockPromises.reset();
    promise1 = Q("foo");
    promise2 = Q("bar");
  });
  it("calls handlers for that promise synchronously", function() {
    var promisedValue;
    promise1.then(function(value) {
      promisedValue = value;
    });
    promise2.then(function(value) {
      promisedValue = value;
    });
    promisedValue = "not foo";
    mockPromises.executeForPromise(promise1);
    expect(promisedValue).toEqual("foo");
  });
});
 
describe("iterateForPromise", function() {
 it('calls the next generation of handlers if the promise has been executed', function() {
    var parentValue = 'not foo';
    promise1 = PromiseWrapper('foo');
    promise2 = promise1.then(function(value) {
      parentValue = value;
      return value + 'bar';
    });
    var childValue1 = 'not foobar'
    var childValue2 = 'not foobar';
    promise2.then(function(value) {
      childValue1 = value;
    });
    promise2.then(function(value) {
      childValue2 = value;
    });
    mockPromises.executeForPromise(promise1);
    expect(parentValue).toEqual("foo");
    expect(childValue1).toEqual("not foobar");
    expect(childValue2).toEqual("not foobar");
    mockPromises.iterateForPromise(promise1);
    expect(parentValue).toEqual("foo");
    expect(childValue1).toEqual("foobar");
    expect(childValue2).toEqual("foobar");
  });
});