Synchronous Promise stubbing for Sinon.JS
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README.md

sinon-stub-promise

Build Status

This is a little package that makes testing of promises easier when stubbing with Sinon.JS. This library ensures stubbed promises are evaluated synchronously, so that no special async tricks are required for testing.

Installation

Install with npm: npm install --save-dev sinon-stub-promise

In node, you can initialize with sinon:

var sinon = require('sinon');
var sinonStubPromise = require('sinon-stub-promise');
sinonStubPromise(sinon);

Or in the browser, you can just include node_modules/sinon-stub-promise/index.js (assumes sinon is available on window object).

Example

// Code under test
function doSomethingWithAPromise(promise, object) {
  promise()
    .then(function(value) {
      // resolves
      object.resolved = value
    })
    .catch(function(value) {
      // rejects
      object.rejected = value
    });
}

// Test
describe('stubbing a promise', function() {
  var promise;

  beforeEach(function() {
    promise = sinon.stub().returnsPromise();
  });

  it('can resolve', function() {
    promise.resolves('resolve value')

    var testObject = {};
    doSomethingWithAPromise(promise, testObject);
    expect(testObject.resolved).to.eql('resolve value');
  });

  it('can reject', function() {
    promise.rejects('reject value')

    var testObject = {};
    doSomethingWithAPromise(promise, testObject);
    expect(testObject.rejected).to.eql('reject value');
  });
}

Why?

We wanted a nice synchronous way of stubbing out promises while testing, and the existing solution, sinon-as-promised, uses a promise under the hood to achieve the stubbing. The issue with this, is that the promise is evaluated asynchronously, so the test code has to deal with that by delaying the assertion until the promise has a chance to run.

Additionally, sinon-as-promised requires you to call either stub.resolves() or stub.rejects() before it will setup the stub as a "thenable" object (one that has then and catch on it). The trouble with this is that if you are testing conditional branches (e.g. test what happens when promise succeeds, then test what happens when promise fails), you have to either resolve or reject the promise for the code under test to pass.

Usage with Karma

In order to use this with the Karma test runner you can either add node_modules/sinon-stub-promise/index.js to files in your config or, alternatively, use the karma-sinon-stub-promise plugin.

Stability?

This is not a Promises/A+ compliant library. We built it to support how we are currently using promises. There is a test suite that will grow over time as we identify any short comings of this library.

To Do

  • Allow for chaining with withArgs. Ideally we could do things like sinon.stub().withArgs(42).resolves('value').