How to Dynamically Change Jest Mock Function Return Values in Tests

How to Dynamically Change Jest Mock Function Return Values in Tests

17 December 2024 Stephan Petzl Leave a comment Tech-Help

When testing components in Jest, you might encounter a scenario where you need to alter the return values of mock functions for each test. This is particularly useful when you want to create snapshots based on different configurations of these mock functions. In this guide, we will explore how to effectively change the return values of Jest mock functions across different tests.

Understanding Mock Functions in Jest

Jest provides a way to mock modules and functions, allowing you to replace them with mock implementations. This is achieved using the jest.mock() function, which can create mock functions using jest.fn(). Once these mocks are established, you can utilize methods such as mockImplementation() and mockReturnValueOnce() to control their behavior in your tests.

Step-by-Step Guide

  1. Mock the Module: Begin by mocking the module and its functions using jest.fn(). 
    import { navigationEnabled, guidanceEnabled } from '../../../magic/index';

jest.mock('../../../magic/index', () => ({
  navigationEnabled: jest.fn(),
  guidanceEnabled: jest.fn()
}));
  2. Set Return Values for Each Test: Use mockImplementation() or mockReturnValueOnce() within your test cases to alter the return values dynamically. 
    describe('Test Suite', () => {
  it('should return true for navigationEnabled', () => {
    navigationEnabled.mockReturnValueOnce(true);
    // Test assertions here
  });

  it('should return false for navigationEnabled', () => {
    navigationEnabled.mockReturnValueOnce(false);
    // Test assertions here
  });
});

Practical Example

In the example above, we set different return values for the navigationEnabled mock function in each test case. This allows us to test various scenarios without modifying the original module.

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