Promises in JavaScript Unit Tests: the Definitive Guide

JavaScript promises simplify asynchronous coding, but testing them can be tricky. This guide clarifies how to effectively handle promises within unit tests, avoiding common pitfalls and improving code readability. A sample project illustrating these techniques is available on the author's website (link not provided in original text).

Key Takeaways:

• Promise handling in unit tests can be complex, leading to false positives if not managed carefully.
• Mocha's built-in promise support automatically fails tests on rejected promises, simplifying the process.
• chai-as-promised enables direct promise assertions, enhancing test clarity.
• Always return a promise from your test function to ensure Mocha recognizes and handles it correctly.

Getting Started (Mocha & Chai):

Install Mocha and Chai:

npm install mocha chai

A naive approach to testing promises often results in verbose, less readable tests:

var expect = require('chai').expect;

it('should do something with promises', function(done) {
  var blah = 'foo';
  var result = systemUnderTest();
  result.then(function(data) {
    expect(data).to.equal(blah);
    done();
  }, function(error) {
    assert.fail(error);
    done();
  });
});

The done() callback and error handling add unnecessary complexity. Without proper error handling, a rejected promise could lead to a false positive.

Mocha and Promises:

Mocha's built-in promise support simplifies this:

it('should fail the test', function() {
  return Promise.reject('this promise will always be rejected');
});

A rejected promise automatically fails the test. Our initial example can be improved:

var expect = require('chai').expect;

it('should do something with promises', function() {
  var blah = 'foo';
  var result = systemUnderTest();
  return result.then(function(data) {
    expect(data).to.equal(blah);
  });
});

Returning the promise eliminates the need for done() and explicit error handling.

Improving Tests with chai-as-promised:

Install chai-as-promised:

npm install chai-as-promised

This library allows for more concise assertions:

var chai = require('chai');
var expect = chai.expect;
var chaiAsPromised = require('chai-as-promised');
chai.use(chaiAsPromised);

it('should do something with promises', function() {
  var blah = 'foo';
  var result = systemUnderTest();
  return expect(result).to.eventually.equal(blah);
});

The eventually keyword handles the promise's asynchronous nature. Remember to always return the promise. Various Chai assertions work with eventually.

Useful Patterns:

• Comparing Objects: Use eventually.deep.equal or eventually.become for deep object comparisons.
• Specific Object Properties: Chain promises to access and assert on specific properties. ES6 arrow functions can improve readability.
• Multiple Promises: Use Promise.all to handle multiple promises concurrently (but be mindful of potential code smells from multiple assertions in a single test).
• Comparing Multiple Promises: Use Promise.all to resolve promises and then compare results.
• Asserting Failures: Use to.be.rejected and to.be.rejectedWith for rejection assertions.
• Test Hooks: Promises work seamlessly in Mocha's before, after, beforeEach, and afterEach hooks.

Promises and Mocks/Stubs (with Sinon.JS):

Install Sinon.JS:

npm install mocha chai

Use sinon.stub().returns(Promise.resolve/reject(...)) to create stubs returning promises. Consider sinon-as-promised for simplified promise stubbing.

Conclusions:

With Mocha, Chai, and chai-as-promised, testing promises becomes significantly cleaner and more readable. Always return promises from your test functions. The provided sample project (link not available) offers practical examples.

Frequently Asked Questions (FAQs): (The original FAQs are omitted due to length and redundancy. They largely cover the information already present in the article.)

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