Both are test doubles that stand-ins for when developers don’t want to use the real thing during test.
1. Stubs/Spies/Mocks
A stub is a type of test double that replaces a specific function or method with a simplified implementation. In Jest they are instantiated with jest.fn() and used with expect(stub).<assertionName>
const myFunction = jest.fn().mockReturnValue(42);Spy focuses on observing the behavior of a function rather than replacing it. In Jest you can use jest.spyOn(). The function wrap the existing object/function method and allow you to observes it’s behavior.
const mockEventListenerOnMessage = jest
.spyOn(window, "addEventListener")
.mockImplementationOnce((event, handler) => {})Spy can test the behavior and state that happen internally.
We can use .mockImplementation() or mockReturnValue/mockResolvedValue to replace a function’s implementation.
const myFunction = jest.fn();
myFunction.mockImplementation(() => {
return 'Custom implementation';
});We also can mock a module like this example:
// Create a mock implementation for a module
jest.mock('./myModule');
// Import the mocked module
const myModule = require('./myModule');
// Provide a mock implementation that returns data
myModule.getData.mockReturnValue({ name: 'Jane', age: 25 });
// Call the function from the mocked module
const result = myModule.getData();
// Verify the returned data
expect(result).toEqual({ name: 'Jane', age: 25 });2. Behavior vs State testing
Behavior:
Function testing (black-box) -> Focus on verifying the behavior of your code. You ensure that your code behaves correctly and consistently based on its inputs and expected outcomes.
State:
Unit testing (white-box) -> Focus on testing the internal state or implementation details of your code. (Like a counter app that should increase the counter value when count function was called)
Mainly the distinction between them comes from the testing approach rather than specific Jest functions.
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