testdouble.js

Welcome! Are you writing JavaScript tests and in the market for a mocking library to fake out real things for you? testdouble.js is an opinionated, carefully-designed test double library maintained by, oddly enough, a software agency that's also named Test Double.

If you practice test-driven development, testdouble.js was designed to promote terse, clear, and easy-to-understand tests. There's an awful lot to cover, so please take some time and enjoy our documentation, which itself is designed to show you how to make the most out of test doubles in your tests.

The pitch

Interested in learning what testdouble.js is, why it exists, and what the API offers? The quickest path is this fast-paced 20-minute talk:

Coming from Sinon.js?

Right now, Sinon.js is the test double incumbent in JavaScript, with over 1.7 million downloads in the last month. If you've got experience with Sinon, check out our side-by-side comparison to see why we wrote testdouble.js and how some of the API translates.

The Very Basics

Before diving into our in-depth docs, here is a quick intro of the basic uses:

Stubbing return values for functions

var td = require('testdouble');

var fetch = td.function();
td.when(fetch(42)).thenReturn('Jane User');

fetch(42); // -> 'Jane User'

Verifying a function was invoked

var td = require('testdouble');

var save = td.function('.save');
save(41, 'Jane');

td.verify(save(41, 'Jill'));
//
// Error: Unsatisfied verification on test double `.save`.
//
//   Wanted:
//     - called with `(41, "Jill")`.
//
//   But was actually called:
//     - called with `(41, "Jane")`.

Docs

All of our docs are in the docs/ directory inside this repository and numbered for easy reading in the priority-order we anticipate people needing them. Here's a rough outline:

1. Installation
2. for Node.js
3. for browsers
4. initial configuration
5. Purpose of testdouble.js
6. in unit tests
7. in integration tests
8. Getting started tutorial
9. Creating test doubles
10. test double functions with td.function()
11. test double objects with td.object() 1. objects that mirror a constructor function 2. objects that mirror an object of functions 3. object of functions for an array of names 4. object of any functions using ES2015 Proxy
12. Stubbing responses
13. td.when() API
14. equality argument matching
15. one-liner stubbings
16. stubbing sequential return values
17. argument matchers 1. td.matchers.anything() 2. td.matchers.isA() 3. td.matchers.contains()
    1. matching strings
    2. matching arrays
    3. matching objects 4. td.matchers.argThat() 5. td.matchers.not()
18. Stubbing callback APIs
19. Stub exceptions with thenThrow
20. Stub promises with thenResolve and thenReject
21. Stub side effects with thenDo
22. Configuring stubbings 1. ignoreExtraArgs 2. times
23. Verifying invocations
24. td.verify() API
25. equality argument matching
26. argument matchers 1. td.matchers.anything() 2. td.matchers.isA() 3. td.matchers.contains()
    1. matching strings
    2. matching arrays
    3. matching objects 4. td.matchers.argThat()
27. Argument captors
28. Configuring verifications 1. ignoreExtraArgs 2. times
29. Replacing dependencies with test doubles
30. for Node.js
31. for Browser JS
32. td.replace() API
33. Writing custom argument matchers
34. Debugging with testdouble.js
35. td.explain() API
36. Plugins
37. testdouble-chai
38. testdouble-jasmine
39. Frequently Asked Questions
40. Why doesn't td.replace() work with external CommonJS modules?
41. Configuration
42. td.config