jest reset spy with code examples

Jest is a popular testing framework used in JavaScript applications. One of the most powerful features of Jest is its ability to create spies, which are functions that track and report information about their calls.

However, sometimes it is necessary to reset these spies so that they do not interfere with subsequent tests. The Jest reset spy feature allows you to reset all spies, or individual spies, to their original state. In this article, we will explore Jest reset spy with code examples.

Creating a Jest Spy

Before we dive into resetting Jest spies, let's briefly discuss how to create a Jest spy. A Jest spy is created using the jest.spyOn() function. This function takes two arguments: the object or module you would like to spy on, and the name of the function you would like to spy on.

const objectToSpyOn = {
  foo: () => 'bar',
};

const spy = jest.spyOn(objectToSpyOn, 'foo');

In the example above, we're creating a spy for the foo function on the objectToSpyOn object. We can now call spy() instead of objectToSpyOn.foo() to track information about the call.

Resetting All Spies

The Jest reset spy feature allows you to reset all spies by calling the jest.restoreAllMocks() function. This function restores all mocks that have been created using jest.spyOn() or jest.fn().

jest.restoreAllMocks();

The above code restores all mocks to their original state. This is useful when you want to ensure that subsequent tests are not affected by previously created spies.

Resetting Individual Spies

Sometimes, you only want to reset a specific spy. This can be accomplished by calling the mockRestore() function on the spy you would like to reset.

const objectToSpyOn = {
  foo: () => 'bar',
};

const spy = jest.spyOn(objectToSpyOn, 'foo');

// Reset individual spy
spy.mockRestore();

In the example above, we're creating a spy for the foo function on the objectToSpyOn object. We can reset this spy by calling spy.mockRestore().

Using Jest reset spy is important to ensure that your tests are not affected by previously created spies. It is especially important if you are running tests in parallel as these spies can cause unexpected behavior.

Conclusion

In conclusion, Jest reset spy is an important feature that allows you to reset all spies or specific spies to their original state. This ensures that subsequent tests are not affected by previously created spies. When creating spies using jest.spyOn(), always remember to reset them before running subsequent tests. By using Jest reset spy, you can ensure that your tests are accurate and reliable.

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Creating a Jest Spy

When creating a Jest spy, you can also provide an implementation for the function being spied on using the .mockImplementation() method. This method takes a function that will be called instead of the original implementation.

const objectToSpyOn = {
  foo: () => 'bar',
};

const spy = jest.spyOn(objectToSpyOn, 'foo');

spy.mockImplementation(() => 'baz');

console.log(objectToSpyOn.foo()); // 'baz'

In the example above, we're creating a spy for the foo function on the objectToSpyOn object. We're then setting the implementation for the spy to return 'baz'. When we call objectToSpyOn.foo(), the spy's implementation is used and 'baz' is returned instead of 'bar'.

Using .mockImplementation() is useful when you need to test how your application handles different return values or error conditions.

Resetting Individual Spies

When resetting individual spies using spy.mockRestore(), Jest will remove the spy and restore the original implementation of the spied function or method. However, you should keep in mind that this will only work if the original function or method was a part of an object or a module.

If you're spying on a standalone function, you can reset the spy by setting its implementation to undefined.

const standaloneFunction = () => 'foo';

const spy = jest.spyOn(standaloneFunction);

spy.mockImplementation(undefined);

console.log(standaloneFunction()); // 'foo'

In the example above, we're creating a spy for the standaloneFunction. We're then setting the implementation for the spy to undefined, which effectively removes the spy. When we call standaloneFunction(), the original implementation is used and 'foo' is returned.

Using .mockImplementation(undefined) is useful when you're testing a standalone function and need to remove the spy without affecting the original function.

Conclusion

In this article, we've explored Jest reset spy in more detail. We've seen how to create Jest spies, how to set their implementation, and how to reset them individually or all at once. By using Jest reset spy, you can ensure that your tests are accurate and reliable and that they're not affected by previously created spies.

Popular questions

1. What is a Jest spy?
   A Jest spy is a function created using jest.spyOn() to track and report information about function calls in Jest tests.

2. How can you reset all spies in Jest?
   You can reset all spies in Jest by calling the jest.restoreAllMocks() function. This restores all mocks created using jest.spyOn() or jest.fn().

3. How can you reset an individual spy in Jest?
   You can reset an individual spy by calling the mockRestore() function on the spy you would like to reset.

4. How can you set the implementation for a Jest spy?
   You can set the implementation for a Jest spy using the .mockImplementation() method. This takes a function that will be called instead of the spied function's original implementation.

5. What is the purpose of Jest reset spy?
   The purpose of Jest reset spy is to ensure that subsequent tests in a Jest test suite are not affected by previously created spies. By resetting spies, you can ensure that your tests are accurate and reliable.

Tag

Spying.