A minimal test double library for TDD with JavaScript

Usage no npm install needed!

<script type="module">
  import testdouble from 'https://cdn.skypack.dev/testdouble';


testdouble.js (AKA td.js)

Build Status npmjs unpkg

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. (The term "test double" was coined by Gerard Meszaros in his book xUnit Test Patterns.)

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 is designed to show you how to make the most out of test doubles in your tests.

This library was designed to work for both Node.js and browser interpeters. It's also test-framework agnostic, so you can plop it into a codebase using Jasmine, Mocha, Tape, Jest, or our own teenytest.


$ npm install -D testdouble

If you just want to fetch the browser distribution, you can also curl it from unpkg.

We recommend requiring the library in a test helper and setting it globally for convenience to the shorthand td:

// ES import syntax
import * as td from 'testdouble'

// CommonJS modules (e.g. Node.js)
global.td = require('testdouble')

// Global set in our browser distribution

(You may need to configure your linter to ignore the td global. Instructions: eslint, standard.)

If you're using testdouble.js in conjunction with another test framework, you may also want to check out one of these extensions:

Getting started

Mocking libraries are more often abused than used effectively, so figuring out how to document a mocking library so as to only encourage healthy uses has proven to be a real challenge. Here are a few paths we've prepared for getting started with testdouble.js:

Of course, if you're unsure of how to approach writing an isolated test with testdouble.js, we welcome you to open an issue on GitHub to ask a question.


td.replace() and td.replaceEsm() for replacing dependencies

The first thing a test double library needs to do is give you a way to replace the production dependencies of your subject under test with fake ones controlled by your test.

We provide a top-level function called td.replace() that operates in two different modes: CommonJS module replacement and object-property replacement. Both modes will, by default, perform a deep clone of the real dependency which replaces all functions it encounters with fake test double functions which can, in turn, be configured by your test to either stub responses or assert invocations.

Additionally, if you're using Node 13 or newer, you can specify testdouble as a module loader and replace native ES modules with td.replaceEsm(). More details here.

Module replacement with Node.js

td.replace('../path/to/module'[, customReplacement])

If you're using Node.js and don't mind using the CommonJS require() function in your tests (you can still use import/export in your production code, assuming you're compiling it down for consumption by your tests), testdouble.js uses a library we wrote called quibble to monkey-patch require() so that your subject will automatically receive your faked dependencies simply by requiring them. This approach may be familiar if you've used something like proxyquire, but our focus was to enable an even more minimal test setup.

Here's an example of using td.replace() in a Node.js test's setup:

let loadsPurchases, generatesInvoice, sendsInvoice, subject
module.exports = {
  beforeEach: () => {
    loadsPurchases = td.replace('../src/loads-purchases')
    generatesInvoice = td.replace('../src/generates-invoice')
    sendsInvoice = td.replace('../src/sends-invoice')
    subject = require('../src/index')
  afterEach: function () { td.reset() }

In the above example, at the point when src/index is required, the module cache will be bypassed as index is loaded. If index goes on to subsequently require any of the td.replace()'d dependencies, it will receive a reference to the same fake dependencies that were returned to the test.

Because td.replace() first loads the actual file, it will do its best to return a fake that is shaped just like the real thing. That means that if loads-purchases exports a function, a test double function will be created and returned. If generates-invoice exports a constructor, a constructor test double will be returned, complete with test doubles for all of the original's static functions and instance methods. If sends-invoice exports a plain object of function properties, an object will be returned with test double functions in place of the originals' function properties. In every case, any non-function properties will be deep-cloned.

There are a few important things to keep in mind about replacing Node.js modules using td.replace():

  • The test must td.replace() and require() everything in a before-each hook, in order to bypass the Node.js module cache and to avoid pollution between tests
  • Any relative paths passed to td.replace() are relative from the test to the dependency. This runs counter to how some other tools do it, but we feel it makes more sense
  • The test suite (usually in a global after-each hook) must call td.reset() to ensure the real require() function and dependency modules are restored after each test case.
Default exports with ES modules

If your modules are written in the ES module syntax and they specify default exports (e.g. export default function loadsPurchases()), but are actually transpiled to CommonJS, just remember that you'll need to reference .default when translating to the CJS module format.

That means instead of this:

loadsPurchases = td.replace('../src/loads-purchases')

You probably want to assign the fake like this:

loadsPurchases = td.replace('../src/loads-purchases').default

Property replacement

td.replace(containingObject, nameOfPropertyToReplace[, customReplacement])

If you're running tests outside Node.js or otherwise injecting dependencies manually (or with a DI tool like dependable), then you may still use td.replace to automatically replace things if they're referenceable as properties on an object.

To illustrate, suppose our subject depends on app.signup below:

app.signup = {
  onSubmit: function () {},
  onCancel: function () {}

If our goal is to replace app.signup during a test of app.user.create(), our test setup might look like this:

let signup, subject
module.exports = {
  beforeEach: function () {
    signup = td.replace(app, 'signup')
    subject = app.user
  // …
  afterEach: function () { td.reset() }

td.replace() will always return the newly-created fake imitation, even though in this case it's obviously still referenceable by the test and subject alike with app.signup. If we had wanted to only replace the onCancel function for whatever reason (though in this case, that would smell like a partial mock), we could have called td.replace(app.signup, 'onCancel'), instead.

Remember to call td.reset() in an after-each hook (preferably globally so one doesn't have to remember to do so in each and every test) so that testdouble.js can replace the original. This is crucial to avoiding hard-to-debug test pollution!

Specifying a custom replacement

The library's imitation feature is pretty sophisticated, but it's not perfect. It's also going to be pretty slow on large, complex objects. If you'd like to specify exactly what to replace a real dependency with, you can do so in either of the above modes by providing a final optional argument.

When replacing a Node.js module:

generatesInvoice = td.replace('../generates-invoice', {
  generate: td.func('a generate function'),
  name: 'fake invoices'

When replacing a property:

signup = td.replace(app, 'signup', {
  onSubmit: td.func('fake submit handler'),
  onCancel: function () { throw Error('do not call me') }

td.func(), td.object(), td.constructor(), td.instance() and td.imitate() to create test doubles

td.replace()'s imitation and injection convenience is great when your project's build configuration allows for it, but in many cases you'll want or need the control to create fake things directly. Each creation function can either imitate a real thing or be specified by passing a bit of configuration.

Each test double creation function is very flexible and can take a variety of inputs. What gets returned generally depends on the number and type of configuration parameters passed in, so we'll highlight each supported usage separately with an example invocation:


The td.func() function (also available as td.function()) returns a test double function and can be called in three modes:

  • td.func(someRealFunction) - returns a test double function of the same name, including a deep imitation of all of its custom properties
  • td.func() - returns an anonymous test double function that can be used for stubbing and verifying any calls against it, but whose error messages and debugging output won't have a name to trace back to it
  • td.func('some name') - returns a test double function named 'some name', which will appear in any error messages as well as the debug info returned by passing the returned test double into td.explain()
  • td.func<Type>() - returns a test double function imitating the passed type. Examples and more details can be found in using with TypeScript


The td.object() function returns an object containing test double functions, and supports three types of invocations:

  • td.object(realObject) - returns a deep imitation of the passed object, where each function is replaced with a test double function named for the property path (e.g. If realObject.invoices.send() was a function, the returned object would have property invoices.send set to a test double named '.invoices.send')
  • td.object(['add', 'subtract']) - returns a plain JavaScript object containing two properties add and subtract that are both assigned to test double functions named '.add' and '.subtract', respectively
  • td.object('a Person'[, {excludeMethods: ['then']}) - when passed with no args or with a string name as the first argument, returns an ES Proxy. The proxy will automatically intercept any call made to it and shunt in a test double that can be used for stubbing or verification. More details can be found in our full docs
  • td.object<Interface>() - returns an object with methods exposed as test doubles that are typed according to the passed interface. Examples and more details can be found in using with TypeScript


If your code depends on ES classes or functions intended to be called with new, then the td.constructor() function can replace those dependencies as well.

  • td.constructor(RealConstructor) - returns a constructor whose calls can be verified and whose static and prototype functions have all been replaced with test double functions using the same imitation mechanism as td.func(realFunction) and td.object(realObject)
  • td.constructor(['select', 'save']) - returns a constructor with select and save properties on its prototype object set to test double functions named '#select' and '#save', respectively

When replacing a constructor, typically the test will configure stubbing & verification by directly addressing its prototype functions. To illustrate, that means in your test you might write:

const FakeConstructor = td.constructor(RealConstructor)


So that in your production code you can:

const subject = function (SomeConstructor) {
  const thing = new SomeConstructor()
  return thing.doStuff() // returns "ok"


As a shorthand convenience, td.instance() function will call td.constructor() and return a new instance of the fake constructor function it returns.

The following code snippets are functionally equivalent:

const fakeObject = td.instance(RealConstructor)
const FakeConstructor = td.constructor(RealConstructor)
const fakeObject = new FakeConstructor()


td.imitate(realThing[, name])

If you know you want to imitate something, but don't know (or care) whether it's a function, object, or constructor, you can also just pass it to td.imitate() with an optional name parameter.

td.when() for stubbing responses

td.when(__rehearsal__[, options])

Once you have your subject's dependencies replaced with test double functions, you'll want to be able to stub return values (and other sorts of responses) when the subject invokes the test double in the way that the test expects.

To make stubbing configuration easy to read and grep, td.when()'s first argument isn't an argument at all, but rather a placeholder to demonstrate the way you're expecting the test double to be invoked by the subject, like so:

const increment = td.func()

We would say that increment(5) is "rehearsing the invocation". Note that by default, a stubbing is only satisfied when the subject calls the test double exactly as it was rehearsed. This can be customized with argument matchers, which allow for rehearsals that do things like increment(td.matchers.isA(Number)) or save(td.matchers.contains({age: 21})).

Also note that, td.when() takes an optional configuration object as a second parameter, which enables advanced usage like ignoring extraneous arguments and limiting the number of times a stubbing can be satisfied.

Calling td.when() returns a number of functions that allow you to specify your desired outcome when the test double is invoked as demonstrated by your rehearsal. We'll begin with the most common of these: thenReturn.


td.when(__rehearsal__[, options]).thenReturn('some value'[, more, values])

The simplest example is when you want to return a specific value in exchange for a known argument, like so:

const loadsPurchases = td.replace('../src/loads-purchases')
td.when(loadsPurchases(2018, 8)).thenReturn(['a purchase', 'another'])

Then, in the hands of your subject under test:

loadsPurchases(2018, 8) // returns `['a purchase', 'another']`
loadsPurchases(2018, 7) // returns undefined, since no stubbing was satisfied

If you're not used to stubbing, it may seem contrived to think a test will know exactly what argument to pass in and expect back from a dependency, but in an isolated unit test this is not only feasible but entirely normal and expected! Doing so helps the author ensure the test remains minimal and obvious to future readers.

Note as well that subsequent matching invocations can be stubbed by passing additional arguments to thenReturn(), like this:

const hitCounter = td.func()
td.when(hitCounter()).thenReturn(1, 2, 3, 4)

hitCounter() // 1
hitCounter() // 2
hitCounter() // 3
hitCounter() // 4
hitCounter() // 4

td.when().thenResolve() and td.when().thenReject()

td.when(__rehearsal__[, options]).thenResolve('some value'[, more, values])

td.when(__rehearsal__[, options]).thenReject('some value'[, more, values])

The thenResolve() and thenReject() stubbings will take whatever value is passed to them and wrap it in an immediately resolved or rejected promise, respectively. By default testdouble.js will use whatever Promise is globally defined, but you can specify your own like this:

td.config({promiseConstructor: require('bluebird')})`

Because the Promise spec indicates that all promises must tick the event loop, keep in mind that any stubbing configured with thenResolve or thenReject must be managed as an asynchronous test (consult your test framework's documentation if you're not sure).


td.when(__rehearsal__[, options]).thenCallback('some value'[,other, args])

The thenCallback() stubbing will assume that the rehearsed invocation has an additional final argument that takes a callback function. When this stubbing is satisfied, testdouble.js will invoke that callback function and pass in whatever arguments were sent to thenCallback().

To illustrate, consider this stubbing:

const readFile = td.replace('../src/read-file')
td.when(readFile('my-secret-doc.txt')).thenCallback(null, 'secrets!')

Then, the subject might invoke readFile and pass an anonymous function:

readFile('my-secret-doc.txt', function (err, contents) {
  console.log(contents) // will print 'secrets!'

If the callback isn't in the final position, or if the test double also needs to return something, callbacks can be configured using the td.callback argument matcher.

On one hand, thenCallback() can be a great way to write fast and clear synchronous isolated unit tests of production code that's actually asynchronous. On the other hand, if it's necessary to verify the subject behaves correctly over multiple ticks of the event loop, you can control this with the defer and delay options.


td.when(__rehearsal__[, options]).thenThrow(new Error('boom'))

The thenThrow() function does exactly what it says on the tin. Once this stubbing is configured, any matching invocations will throw the specified error.

Note that because rehearsal calls invoke the test double function, it's possible to configure a thenThrow stubbing and then accidentally trigger it when you attempt to configure subsequent stubbings or verifications. In these cases, you'll need to work around it by re-ordering your configurations or catch'ing the error.


td.when(__rehearsal__[, options]).thenDo(function (arg1, arg2) {})

For everything else, there is thenDo(). thenDo takes a function which will be invoked whenever satisfied with all the arguments and bound to the same this context that the test double function was actually invoked with. Whatever your thenDo function returns will be returned by the test double when the stubbing is satisfied. This configuration is useful for covering tricky cases not handled elsewhere, and may be a potential extension point for building on top of the library's stubbing capabilities.

td.verify() for verifying interactions

td.verify(__demonstration__[, options])

If you've learned how to stub responses with td.when() then you already know how to verify an invocation took place with td.verify()! We've gone out of our way to make the two as symmetrical as possible. You'll find that they have matching function signatures, support the same argument matchers, and take the same options.

The difference, then, is their purpose. While stubbings are meant to facilitate some behavior we want to exercise in our subject, verifications are meant to ensure a dependency was called in a particular expected way. Since td.verify() is an assertion step, it goes at the end of our test after we've invoked the subject under test.

A trivial example might be:

module.exports = function shouldSaveThings () {
  const save = td.replace('../src/save')
  const subject = require('../src/index')

  subject({name: 'dataz', data: '010101'})

  td.verify(save('dataz', '010101'))

The above will verify that save was called with the two specified arguments. If the verification fails (say it passed '010100' instead), testdouble.js will throw a nice long error message to explain how the test double function was actually called, hopefully helping you spot the error.

Just like with td.when(), more complex cases can be covered with argument matchers and configuration options.

A word of caution: td.verify() should be needed only sparingly. When you verify a function was called (as opposed to relying on what it returns) you're asserting that your subject has a side effect. Code with lots of side effects is bad, so mocking libraries are often abused to make side-effect heavy code easier to proliferate. In these cases, refactoring each dependency to return values instead is almost always the better design approach. A separate test smell with verifying calls is that sometimes—perhaps in the interest of maximal completeness—a test will verify an invocation that already satisfied a stubbing, but this is almost provably unnecessary.

Other functions

For other top-level features in the testdouble.js API, consult the docs directory: