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Creating Test Doubles

Unlike most classical or functional languages, JavaScript under test might depend on many sorts of things: plain objects, instantiable constructor functions, ES2015 classes, or regular old functions. As a result, any good test double library needs to provide terse and convenient ways to create fake versions of all of these things.

The examples below will walk through the different ways to create new test doubles. Each example assumes you've aliased testdouble to td.

This document discusses all the ways to manually create test doubles, but keep in mind that in practice many developers will lean on td.replace, which performs this duty on your behalf. Read more about td.replace on its doc page.


Note that td.func is available as an alias of td.function.

To create a fake function with test double, we use the function function. At its simplest, invoking:

var bark = td.function() // '[test double (unnamed)]'

bark is now a test double function, meaning it can be configured to stub a particular response with td.when, its invocations can be verified with td.verify, and its current state can be introspected with td.explain.

To provide yourself with better messages, we recommend assigning a name to the function; this is particularly important whenever a test will have more than one double in use at a time:

var woof = td.function('.woof') // test double for ".woof"

Creating a test double function from a real function

If you're replacing an actual function with a test double, you can also pass it to td.function and all of its properties will be copied over. If any of its properties are functions, those functions will be replaced with test double functions as well.

function meow () { throw 'unimplemented' }
meow.volume = 'loud'
meow.stop = function () {}

var fakeMeow = td.function(meow) // test double for "meow"
fakeMeow.volume // 'loud'
fakeMeow.stop // test double for "meow.stop"

This can be handy when you're replacing a dependency that is a function but also has significant properties defined (imagine a module that exports a function as well as a synchronous version exposed via a sync property).


Creating a one-off function is really easy, but often our subjects will depend on objects with functions as properties. Because we don't want to encourage the use of partial mocks, test double offers a number of ways to create objects whose function properties have all been replaced with fake functions.

Because there's a number of ways to create objects of test doubles, it's important that we provide some guidance on which tool to use when. Because we use test doubles to practice outside-in test-driven development, most of the fake objects we use represent code that doesn't exist yet. As a result, there are two styles you might take:

  • Pass td.object() a plain JavaScript object which contains functions as properties. testdouble.js will mirror either of these by replacing all the functions it detects. If you practice this style, it means part of your test-driven workflow will require you to define those types in your production code as you write your tests. This is a great way to take advantage of the TDD workflow to implement a skeleton of your object graph as you work, all-the-while keeping the contracts between your subject and its dependencies explicit enough to catch failures when you change the contract between the subject and its dependencies (i.e. if you change the name of a dependency's function, the test double function would disappear and break the caller's test).
  • Pass td.object() an array of function names or (if using a runtime that implements ES2015 Proxy) a name for the object. This style makes prototyping interactions easier because it doesn't require the author to define production-scope functions of dependencies while authoring the caller's test. Conversely, it runs the risk of "fantasy green" tests that continue passing even if a dependency's functions change in the future

No style is right or wrong, as they both have trade-offs (full disclosure: when practicing TDD in a greenfield application, the present author usually prefers the former style). However, we would encourage teams to pick one style and apply it consistently to reduce the cognitive load of keeping straight the expected behavior of tests when depended-on objects change.


Suppose you have an object with some functions as properties (and perhaps some non-function properties). If passed to td.object(), testdouble.js will do a deep copy of the object and replace of any functions found on the object while and return it.

So, given:

var fish = {
  eat: function(){}
  swim: function(){}
  details: {
    age: 10,
    name: 'goldie'

Then you could fake the fish out with:

var fish = td.object(fish) // a test double function named '.eat'
fish.details.age // still `10`


If you pass td.object an array of strings, it'll return a plain object with those properties set as named test double functions.

var cat = td.object(['meow', 'purr'])

cat.meow // a test double function named 'meow'


If passed either a string name or no arguments at all, td.object will return an ES2015 Proxy object designed to forward any property access as if it were a test double function. By using Proxy, testdouble.js is able to intercept calls to properties that don't exist, immediately create a new test double function, and invoke that function for use in either stubbing or verifying behavior.

var parrot = td.object('Parrot')

parrot.squawk // a test double function named 'Parrot#squawk'

Excluding certain methods from Proxy-based test doubles

Sometimes, your subject code will check to see if a property is defined, which may make a bit of code unreachable when a dynamic test double responds to every single accessed property.

For example, if you have this code:

function leftovers(walrus) {
  if(! {
    return 'cheese';

You could create a test double walrus that can reach the cheese with this:

walrus = td.object('Walrus', {excludeMethods: ['eat']});

leftovers(walrus) // 'cheese'

By default, excludeMethods is set to ['then'], so that test libraries like Mocha don't mistake every test double object for a Promise (which would cause the test suite to time out)


Test Double can also create artificial constructor functions. This method can be passed either a constructor (or ES class) or an array of function names and will return a constructor whose functions are all replaced by test doubles.


Suppose you have a constructor function:

function Dog(){}
Dog.prototype.bark = function(){}
Dog.woof = function(){}

To create a test double of the constructor that has prototypal function bark and "static" function woof, simply pass Dog to td.constructor()

var FakeDog = td.constructor(Dog)

FakeDog.prototype.bark // a test double function named 'Dog#bark'
FakeDog.woof // a test double function named 'Dog.woof'

When FakeDog is instantiated, any stubbings (or verifications) set up by the test on the FakeDog.prototype will also translate to the instances, for instance:


var dog = new FakeDog()

dog.bark() // 'YIP


If you want a fake constructable thing to have a certain set of instance methods defined on it, testdouble.js can also create one from an array of function names.

var FakeCat = td.constructor(['meow', 'scratch'])

FakeCat.prototype.meow // a test double function named '#meow'
FakeCat.prototype.scratch // a test double function named '#scratch'


As you can see, there are a plethora of ways to create test doubles with testdouble.js, each designed to handle a different style of organizing JavaScript code. We recommend landing on one consistent style (e.g. each module as one function) for each project, which in turn would encourage one consistent style of creating test doubles. This API is written to be flexible for a number of potential contexts across objects, but it has come at the cost of a large enough surface area that if any project were to make ample use of all or most of the above invocation styles, it would confuse readers.

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