Jest moc

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Jest moc
Last updated on Aug 25, 2022 by Simen Bekkhus
Copyright © 2022 Facebook, Inc. Built with Docusaurus.
Manual mocks are used to stub out functionality with mock data. For example, instead of accessing a remote resource like a website or a database, you might want to create a manual mock that allows you to use fake data. This ensures your tests will be fast and not flaky.
Manual mocks are defined by writing a module in a __mocks__/ subdirectory immediately adjacent to the module. For example, to mock a module called user in the models directory, create a file called user.js and put it in the models/__mocks__ directory. Note that the __mocks__ folder is case-sensitive, so naming the directory __MOCKS__ will break on some systems.
When we require that module in our tests (meaning we want to use the manual mock instead of the real implementation), explicitly calling jest.mock('./moduleName') is required .
If the module you are mocking is a Node module (e.g.: lodash ), the mock should be placed in the __mocks__ directory adjacent to node_modules (unless you configured roots to point to a folder other than the project root) and will be automatically mocked. There's no need to explicitly call jest.mock('module_name') .
Scoped modules (also known as scoped packages ) can be mocked by creating a file in a directory structure that matches the name of the scoped module. For example, to mock a scoped module called @scope/project-name , create a file at __mocks__/@scope/project-name.js , creating the @scope/ directory accordingly.
Warning: If we want to mock Node's core modules (e.g.: fs or path ), then explicitly calling e.g. jest.mock('path') is required , because core Node modules are not mocked by default.
When a manual mock exists for a given module, Jest's module system will use that module when explicitly calling jest.mock('moduleName') . However, when automock is set to true , the manual mock implementation will be used instead of the automatically created mock, even if jest.mock('moduleName') is not called. To opt out of this behavior you will need to explicitly call jest.unmock('moduleName') in tests that should use the actual module implementation.
Note: In order to mock properly, Jest needs jest.mock('moduleName') to be in the same scope as the require/import statement.
Here's a contrived example where we have a module that provides a summary of all the files in a given directory. In this case, we use the core (built in) fs module.
Since we'd like our tests to avoid actually hitting the disk (that's pretty slow and fragile), we create a manual mock for the fs module by extending an automatic mock. Our manual mock will implement custom versions of the fs APIs that we can build on for our tests:
Now we write our test. Note that we need to explicitly tell that we want to mock the fs module because it’s a core Node module:
The example mock shown here uses jest.createMockFromModule to generate an automatic mock, and overrides its default behavior. This is the recommended approach, but is completely optional. If you do not want to use the automatic mock at all, you can export your own functions from the mock file. One downside to fully manual mocks is that they're manual – meaning you have to manually update them any time the module they are mocking changes. Because of this, it's best to use or extend the automatic mock when it works for your needs.
To ensure that a manual mock and its real implementation stay in sync, it might be useful to require the real module using jest.requireActual(moduleName) in your manual mock and amending it with mock functions before exporting it.
The code for this example is available at examples/manual-mocks .
If you're using ES module imports then you'll normally be inclined to put your import statements at the top of the test file. But often you need to instruct Jest to use a mock before modules use it. For this reason, Jest will automatically hoist jest.mock calls to the top of the module (before any imports). To learn more about this and see it in action, see this repo .
If some code uses a method which JSDOM (the DOM implementation used by Jest) hasn't implemented yet, testing it is not easily possible. This is e.g. the case with window.matchMedia() . Jest returns TypeError: window.matchMedia is not a function and doesn't properly execute the test.
In this case, mocking matchMedia in the test file should solve the issue:
This works if window.matchMedia() is used in a function (or method) which is invoked in the test. If window.matchMedia() is executed directly in the tested file, Jest reports the same error. In this case, the solution is to move the manual mock into a separate file and include this one in the test before the tested file:

__tests__/createMockFromModule.test.js
Last updated on Aug 25, 2022 by Simen Bekkhus
Copyright © 2022 Facebook, Inc. Built with Docusaurus.
The jest object is automatically in scope within every test file. The methods in the jest object help create mocks and let you control Jest's overall behavior. It can also be imported explicitly by via import {jest} from '@jest/globals' .
Disables automatic mocking in the module loader.
See automock section of configuration for more information
After this method is called, all require() s will return the real versions of each module (rather than a mocked version).
This is usually useful when you have a scenario where the number of dependencies you want to mock is far less than the number of dependencies that you don't. For example, if you're writing a test for a module that uses a large number of dependencies that can be reasonably classified as "implementation details" of the module, then you likely do not want to mock them.
Examples of dependencies that might be considered "implementation details" are things ranging from language built-ins (e.g. Array.prototype methods) to highly common utility methods (e.g. underscore/lo-dash, array utilities, etc) and entire libraries like React.js.
Returns the jest object for chaining.
Note: this method was previously called autoMockOff . When using babel-jest , calls to disableAutomock will automatically be hoisted to the top of the code block. Use autoMockOff if you want to explicitly avoid this behavior.
Enables automatic mocking in the module loader.
Returns the jest object for chaining.
See automock section of configuration for more information
Note: this method was previously called autoMockOn . When using babel-jest , calls to enableAutomock will automatically be hoisted to the top of the code block. Use autoMockOn if you want to explicitly avoid this behavior.
Also under the alias: .genMockFromModule(moduleName)
Given the name of a module, use the automatic mocking system to generate a mocked version of the module for you.
This is useful when you want to create a manual mock that extends the automatic mock's behavior.
This is how createMockFromModule will mock the following data types:
Creates a new mock function . The new function has no formal parameters and when called will return undefined . This functionality also applies to async functions.
Creates a new class. The interface of the original class is maintained, all of the class member functions and properties will be mocked.
Creates a new deeply cloned object. The object keys are maintained and their values are mocked.
Creates a new empty array, ignoring the original.
Creates a new property with the same primitive value as the original property.
Mocks a module with an auto-mocked version when it is being required. factory and options are optional. For example:
The second argument can be used to specify an explicit module factory that is being run instead of using Jest's automocking feature:
When using the factory parameter for an ES6 module with a default export, the __esModule: true property needs to be specified. This property is normally generated by Babel / TypeScript, but here it needs to be set manually. When importing a default export, it's an instruction to import the property named default from the export object:
The third argument can be used to create virtual mocks – mocks of modules that don't exist anywhere in the system:
Warning: Importing a module in a setup file (as specified by setupFilesAfterEnv ) will prevent mocking for the module in question, as well as all the modules that it imports.
Modules that are mocked with jest.mock are mocked only for the file that calls jest.mock . Another file that imports the module will get the original implementation even if it runs after the test file that mocks the module.
Returns the jest object for chaining.
Writing tests in TypeScript? Use jest.Mocked utility type or jest.mocked() helper method to have your mocked modules typed.
See TypeScript Usage chapter of Mock Functions page for documentation.
See TypeScript Usage chapter of Mock Functions page for documentation.
Indicates that the module system should never return a mocked version of the specified module from require() (e.g. that it should always return the real module).
The most common use of this API is for specifying the module a given test intends to be testing (and thus doesn't want automatically mocked).
Returns the jest object for chaining.
When using babel-jest , calls to mock will automatically be hoisted to the top of the code block. Use this method if you want to explicitly avoid this behavior.
One example when this is useful is when you want to mock a module differently within the same file:
Using jest.doMock() with ES6 imports requires additional steps. Follow these if you don't want to use require in your tests:
Returns the jest object for chaining.
When using babel-jest , calls to unmock will automatically be hoisted to the top of the code block. Use this method if you want to explicitly avoid this behavior.
Returns the jest object for chaining.
Explicitly supplies the mock object that the module system should return for the specified module.
On occasion, there are times where the automatically generated mock the module system would normally provide you isn't adequate enough for your testing needs. Normally under those circumstances you should write a manual mock that is more adequate for the module in question. However, on extremely rare occasions, even a manual mock isn't suitable for your purposes and you need to build the mock yourself inside your test.
In these rare scenarios you can use this API to manually fill the slot in the module system's mock-module registry.
Returns the jest object for chaining.
Note It is recommended to use jest.mock() instead. The jest.mock API's second argument is a module factory instead of the expected exported module object.
Returns the actual module instead of a mock, bypassing all checks on whether the module should receive a mock implementation or not.
Returns a mock module instead of the actual module, bypassing all checks on whether the module should be required normally or not.
Resets the module registry - the cache of all required modules. This is useful to isolate modules where local state might conflict between tests.
Returns the jest object for chaining.
jest.isolateModules(fn) goes a step further than jest.resetModules() and creates a sandbox registry for the modules that are loaded inside the callback function. This is useful to isolate specific modules for every test so that local module state doesn't conflict between tests.
Returns a new, unused mock function . Optionally takes a mock implementation.
See Mock Functions page for details on TypeScript usage.
Determines if the given function is a mocked function.
Creates a mock function similar to jest.fn but also tracks calls to object[methodName] . Returns a Jest mock function .
By default, jest.spyOn also calls the spied method. This is different behavior from most other test libraries. If you want to overwrite the original function, you can use jest.spyOn(object, methodName).mockImplementation(() => customImplementation) or object[methodName] = jest.fn(() => customImplementation);
Since jest.spyOn is a mock. You could restore the initial state calling jest.restoreAllMocks on afterEach method.
Since Jest 22.1.0+, the jest.spyOn method takes an optional third argument of accessType that can be either 'get' or 'set' , which proves to be useful when you want to spy on a getter or a setter, respectively.
Clears the mock.calls , mock.instances , mock.contexts and mock.results properties of all mocks. Equivalent to calling .mockClear() on every mocked function.
Returns the jest object for chaining.
Resets the state of all mocks. Equivalent to calling .mockReset() on every mocked function.
Returns the jest object for chaining.
Restores all mocks back to their original value. Equivalent to calling .mockRestore() on every mocked function. Beware that jest.restoreAllMocks() only works when the mock was created with jest.spyOn ; other mocks will require you to manually restore them.
Instructs Jest to use fake versions of the global date, performance, time and timer APIs. Fake timers implementation is backed by @sinonjs/fake-timers .
Fake timers will swap out Date , performance.now() , queueMicrotask() , setImmediate() , clearImmediate() , setInterval() , clearInterval() , setTimeout() , clearTimeout() with an implementation that gets its time from the fake clock.
In Node environment process.hrtime , process.nextTick() and in JSDOM environment requestAnimationFrame() , cancelAnimationFrame() , requestIdleCallback() , cancelIdleCallback() will be replaced as well.
Calling jest.useFakeTimers() will use fake timers for all tests within the file, until original timers are restored with jest.useRealTimers() .
You can call jest.useFakeTimers() or jest.useRealTimers() from anywhere: top level, inside an test block, etc. Keep in mind that this is a global operation and will affect other tests within the same file. Calling jest.useFakeTimers() once again in the same test file would reset the internal state (e.g. timer count) and reinstall fake timers using the provided options:
For some reason you might have to use legacy implementation of fake timers. It can be enabled like this (additional options are not supported):
Legacy fake timers will swap out setImmediate() , clearImmediate() , setInterval() , clearInterval() , setTimeout() , clearTimeout() with Jest mock functions . In Node environment process.nextTick() and in JSDOM environment requestAnimationFrame() , cancelAnimationFrame() will be also replaced.
Returns the jest object for chaining.
Instructs Jest to restore the original implementations of the global date, performance, time and timer APIs. For example, you may call jest.useRealTimers() inside afterEach hook to restore timers after each test:
Returns the jest object for chaining.
Exhausts the micro -task queue (usually interfaced in node via process.nextTick ).
When this API is called, all pending micro-tasks that have been queued via process.nextTick will be executed. Additionally, if those micro-tasks themselves schedule new micro-tasks, those will be continually exhausted until there are no more micro-tasks remaining in the queue.
Exhausts both the macro -task queue (i.e., all tasks queued by setTimeout() , setInterval() , and setImmediate() ) and the micro -task queue (usually interfaced in node via process.nextTick ).
When this API is called, all pending macro-tasks and micro-tasks will be executed. If those tasks themselves schedule new tasks, those will be continually exhausted until there are no more tasks remaining in the queue.
This is often useful for synchronously executing setTimeouts during a test in order to synchronously assert about some behavior that would only happen after the setTimeout() or setInterval() callbacks executed. See the Timer mocks doc for more information.
Exhausts all tasks queued by setImmediate() .
This function is only available when using legacy fake timers implementation.
Executes only the macro task queue (i.e. all tasks queued by setTimeout() or setInterval() and setImmediate() ).
When this API is called, all timers are advanced by msToRun milliseconds. All pending "macro-tasks" that have been queued via setTimeout() or setInterval() , and would be executed within this time frame will be executed. Additionally, if those macro-tasks schedule new macro-tasks that would be executed within the same time frame, those will be executed until there are no more macro-tasks remaining in the queue, that should be run within msToRun milliseconds.
Executes only the macro-tasks that are currently pending (i.e., only the tasks that have been queued by setTimeout() or setInterval() up to this point). If any of the currently pending macro-tasks schedule new macro-tasks, those new tasks will not be executed by this call.
This is useful for scenarios such as one where the module being tested schedules a setTimeout() whose callback schedules another setTimeout() recursively (meaning the scheduling never stops). In these scenarios, it's useful to be able to run forward in time by a single step at a time.
Advances all timers by the needed milliseconds so that only the next timeouts/intervals will run.
Optionally, you can provide steps , so it will run steps amount of next timeouts/intervals.
Removes any pending timers from the timer system.
This means, if any timers have been scheduled (but have not yet executed), they will be cleared and will never have the opportunity to execute in the future.
Returns the number of fake timers still left to run.
Set the current system time used by fake timers. Simulates a user changing the system clock while your program is running. It affects the current time but it does not in itself cause e.g. timers to fire; they will fire exactly as they would have done without the call to jest.setSystemTime() .
This function is not available when using legacy fake timers implementation.
When mocking time, Date.now() will also be mocked. If you for some reason need access to the real current time, you can invoke this function.
This function is not available when using legacy fake timers implementation.
Set the default timeout interval (in milliseconds) for all tests and before/after hooks in the test file. This only affects the test file from which this function is called.
To set timeout intervals on different tests in the same file, use the timeout option on each individual test .
Note: The default timeout interval is 5 seconds if this method is not called.
Note: If you want to set the timeout for all test files, a good place to do this is in setupFilesAfterEnv .
Runs failed tests n-times until they pass or until the max number of retries is exhausted. options are optional. This only works with the default jest-circus runner! This must live at the top-level of a test file or in a describe block. Retries will not work if jest.retryTimes() is called in a beforeEach or a test block.
If logErrorsBeforeRetry is enabled, Jest will log the error(s) that caused the test to fail to the console, providing visibility on why a retry occurred.
Returns the jest object for chaining.


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