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Complete Guide to Testing React Apps with Jest and Selenium

Custom Matchers for Jest

Learn what a matcher is, why we need them, and how to develop your own.

We'll cover the following...

What is a matcher?

A matcher (or an assertion) is a function that is used to check for a specific condition. Most often, it compares two values. Here are some of the basic matches available in Jest:

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

Why do we need custom matchers?

While Jest is very powerful out-of-box, there is always something specific to the project that you can add. It should improve readability and reduce the amount of code by creating a custom Jest matcher for a common use case.

In one of my projects, I had to test whether a certain value was a correctly formatted ISO date. It can be done with a regular expression, but I had to test it so many times, it was worth moving this logic to a custom Jest matcher.

Add a setup file

Before we add the matcher itself, it is important to add a setup file for Jest. A setup file is a file that is ...