To mock a function with Jest we use the jest.fn() function. This makes our tests more predictable (deterministic) and easier to write. Mock implementations help us control the return values of a function. We use mock functions to override original function behaviour with custom mock implementations. This ability to "spy" on function calls is why mock functions are also called spies. With a mock function we can know the number of times the function was called, the arguments it was called with, the result it returned, and more. Mock functions track how they are called by external code.
Jest clear mocks how to#
Read more about solitary unit tests in the previous article: How to write unit tests in JavaScript with Jest.Ī mock function is a function that replaces the actual implementation of a function with a "fake" (mock) implementation. This unit test is a solitary unit test because the tested unit is isolated from its dependencies. To mock different return values depending on the test we will create a mock function.
IsAtLeast18() will now always return false no matter what we call it with, because the isInteger() mock is set to always return false.īut what about the case when isInteger() returns true? This is why our mock import path is also "./isInteger". The isAtLeast18.js module imports the isInteger.js module under the path "./isInteger". The import path of the mocked module must match the import path that is present in the module we're testing. That function, when called, will always return false. We will mock the imported module with a factory function that behaves just like the default export and returns a function. The isInteger.js module has a single default export - the isInteger() function. We'll start by testing the case of isInteger() returning false.
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Fixing the implementation is not covered by this article, but feel free to play with it as we move through the article.
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We want to see what our tests will teach us about the flaws in our code by passing and failing test cases. We will assume that we're testing a couple of validation rules:Įnter fullscreen mode Exit fullscreen mode We will learn how to mock functions and imported function modules with Jest, and write tests that rely on those mocks to increase the coverage of our test cases. This article will provide an introduction into the concepts behind mocking and how it relates to unit testing. This makes it possible for us to manipulate the control flow of the tested program and reach even those difficult-to-reproduce edge-cases when writing tests. Mock functions are a testing tool that allows us to track how function dependencies are called and control their return values. This is especially true for large code bases where it's tricky to come up with fixtures that will cover all the cases we need to test.īut what if we could control the return value of a function dependency, no matter what arguments it is called with? Writing unit tests for code with dependencies can be difficult. How to write unit tests in JavaScript with Jest
To get the most out of this article, I recommend reading the previous article in the series:
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