Custom Matchers for Jest

Explore how to create and implement custom matchers in Jest to improve the readability and efficiency of your tests. Understand writing matchers that accept arguments and setting up your Jest environment for tailored assertion functions.

We'll cover the following...

- What is a matcher?
- Accept more arguments

JavaScript (JSX)

expect(1).toBe(1); // Checking for value and reference equality
expect({a: 'b'}).toEqual({a: 'b'}); //Checking for deep value equality
expect('abacaba').toMatch(/bac/); // Checking if a string matches a regexp
expect({a: 'b', b: 'c'}).toMatchObject({a: 'b'}); // Checking for a partial object match
expect([1, 2, 3]).toContainEqual(2); // Checking if an array contains an element
expect(2).not.toEqual(3); // using not to negate any matcher
expect({a: 'b'}).toMatchObject({
  a: expect.any(String)
}); // Type checking
const mockFunction = jest.fn(); // Creating a mock function
expect(mockFunction).toHaveBeenCalled(); // Checking if it was called
expect(mockFunction).toHaveBeenCalledWith('abacaba'); // Checking for arguments

1.Introduction

Assessment

2.Write Your First Tests

Assessment

3.More Tests

4.Mocking Modules in Unit Tests

Assessment

5.More on Integration Tests

6.Bonus: Custom Matchers

Custom Matchers for Jest

What is a matcher?

Why do we need custom matchers?