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Replacing Real Dependencies with Test Doubles

All of these test doubles aren't of much value if your subject can't reference them, so testdouble.js provides some conveniences to swapping out your subject's real dependencies for fake ones with its td.replace function.

At present, td.replace provides two mechanisms for providing test doubles to your test subject:

  • Object-property replacement. By passing td.replace(someObject, 'nameOfProperty'), testdouble.js will retain a reference to someObject.nameOfProperty, and replace it during your test
  • Node.js module replacement. By passing td.replace('../path/to/dependency'), testdouble.js will intercept calls to require that dependency module and ensure your subject is handed a fake instead

In both cases, td.replace will grab the real dependency and imitate it with an appropriate fake:

  • Plain functions are replaced with test double functions of the same name, with all properties deep-cloned and any function properties replaced with test double functions
  • Plain objects are deep-cloned and all functions are replaced with test double functions
  • Constructor functions and ES classes with at least one prototypal function defined will be extended by an artificial constructor that will have all the original constructor/class's static & prototypal functions overwritten with test double functions

After the next call to td.reset() (which you should have in an afterEach hook somewhere in your test suite), the real dependencies will all be restored.

If the imitation scheme doesn't suit you or if you don't want your test to drive out the plumbing of defining the dependency, you can pass a final argument to td.replace that will act as the fake that replaces the real dependency (e.g. td.replace('./foo', 42) will monkey-patch require to return 42 for paths that resolve to that absolute path.


This sounds a little like magic, so let's look at a simple example. Suppose we write a simple test in test/lib/car-test.js:

module.exports = {
  beforeEach: function() {
    var brake = td.replace('../../lib/brake')
    var subject = require('../../lib/car')
  'slowing applies the break': function () {


There are few very important things to note about how to use module replacement safely (with great power, etc.):

  • Most importantly: move your replacements and requirements into a beforeEach hook (or equivalent) and be sure you're calling td.reset() after each test case. Because td.replace('../module/path') will disrupt Node's module caching behavior and cause require() to return a fake, it would cause test pollution to keep the require stanzas at the top of the file
  • As a result, your tests will need to use require and not the ES static import keyword. This only applies to your test files, however, you can still feel free to use import in your production source files, where it actually matters if you're leveraging a bundling tool like Webpack or Rollup. Keep in mind that you'll like be doing a lot of td.replace('../path').default assignments if you're using default exports
  • td.replace is designed to be used as part of an outside-in test-driven development workflow, and so calling td.replace for some path will trigger an error until it actually exists and exports the basic shape (e.g. a function, or a bag of functions, or a class) that is expected to be consumed by the subject under test
  • Because td.replace first requires the module being replaced and then performs a deep imitation of whatever the real module exports, any side-effects the to-be-replaced module has will be inadvertently triggered by the test (remember, good modules should be loadable without triggering side effects!)

That's a lot of caveats, but so long as your test and module design is simple and consistent, it's a powerful feature that can drastically simplify the setup of your isolation tests.

Now, in order to make the above test pass, we first need to create lib/brake.js and export a function, so that testdouble.js knows to replace require calls with a test double function (as opposed to a default module.exports object):

module.exports = function(){}

Once the dependency is prototyped, we can implement lib/car.js in such a way as to make our little test pass:

var brake = require('./brake')

module.exports.slowDown = function(){

As you can see, each file listing references the correct relative paths to the replaced dependency. From the perspective of the test, the brake module is in '../../lib/brake', but from the perspective of the subject, brake is in './brake'.

If you'd like to kick the tires on this, you can play with a more involved example project found in the testdouble.js repository. For a more formal discussion of the replace() API, read on.

If you'd like an example of replacing ES classes that use the export keyword, check out the babel example project. (Note again that the test itself must fall back to CommonJS-style require statements, since module replacement requires the dependency be loaded after the replacements are configured, which precludes the use of the static import statement.)

How module replacement works

Under the hood, testdouble.js uses a module called quibble that facilitates td.replace's behavior by monkey-patching Node's require function (specifically, Module._load). When td.replace is invoked for a module, quibble will begin intercepting any require calls made in that file and—bypassing the Node.js module cache—return a test double instead of the actual module that resolves to the same absolute path as whatever path was passed to td.replace.

As a result, keep in mind that you must call td.replace for each of your subject's dependencies before you require your subject itself. If you require your subject before calling td.replace, it will load normally (potentially from the module cache) and any calls to td.replace will be too late to have their intended effect).

Aside: third-party modules

testdouble.js can also replace third-party npm modules. For instance, if you depend on the module is-number, you can, in your test:

var isNumber = td.replace('is-number')
var numbersOnly = require('./numbers-only')
td.when(isNumber('a string')).thenReturn(true) // tee-hee, this is silly

var result = numbersOnly('a string')

assert.equal(result, true)

Should pass for a subject:

var isNumber = require('is-number')

module.exports = function (thing) {
  if (!isNumber(thing)) {
    throw new Error('numbers only!')
  return true

Even though testdouble.js does support replacing third-party npm modules, it is not recommended unless you own the module! Typically, when practicing the sort of outside-in test-driven development that testdouble.js is designed to facilitate, you should keep third-party dependencies at arms-length by only mocking what you own. But if you're managing lots of internal modules and they're all in a consistent style such that the line between first-party & third-party code is blurred, then td.replace has you covered and should be able to replace third-party modules or npm packages just like it can for local paths.


Given the eclectic array of browser JS namespacing, module loading, and packaging schemes (not to mention the limited runtime introspection available to ES import statements), testdouble.js doesn't try solve for every front-end module and packaging scheme. For many projects, the object-property replacement scheme will be good enough. For many others, td.replace could be used in a test helper function to patch module schemes like AMD.

Using global-variable namespacing to carry forward our simple example above, we could write this test:

var brake = td.replace(app, 'brake')
    subject =



First, the test will fail until we define app.brake as a function:

app.brake = function(){}

Now we can write the function to make the test pass: = {
  slowDown: function() {

Once again, it is very important when using this feature that td.reset() is invoked after each test, or else bizarre and terrible things will happen.

td.replace() API

To recap, td.replace has two "modes", for lack of a better term, which result in relatively disparate method signatures. So let's spell those out here:

td.replace(object, propertyName, [manualReplacement])

When td.replace's first argument is anything other than a string, it assumes you're providing the object on which some property should be replaced.

The second argument, propertyName, which must be a string of the same name as some property already defined on object (if undefined properties were allowed to be replaced, minor errors like misspelling would take much longer to catch). It must be a string because td.replace needs to know not just its value, but where to replace it the next time someone calls td.reset().

As discussed at the top, most of the time td.replace will successfully infer the right type of fake thing to set on object and to return. For other cases, you can pass a third manualReplacement argument to specify the value to be set on object[propertyName].

td.replace typically returns the fake thing it sets on object, with the exception of constructor function properties. In that case, it will return a plain object of test double functions to the test, but set object[propertyName] to a constructor function that delegates to those test double functions only after it's been instantiated with new.

td.replace(relativePathToModule, [manualReplacement])

When the first argument to td.replace is a string, it assumes you're replacing a Node.js module dependency and that relativePathToModule is, as its name suggests, a relative path from the test to the module being faked.

In a TDD workflow, this is often the first step to figuring out where that module should reside, and so td.replace will throw an error until it exists and can be loaded. Once defined, it will return a fake thing based on the same inferences discussed above and replace subsequent calls to require for that module until the next call to td.reset()

Also, as mentioned above, if the inference td.replace isn't appropriate or the path shouldn't exist yet, a second argument manualReplacement can be provided to short-circuit any attempts to load and imitate a module at relativePathToModule.

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