Mastering React Test-Driven Development

Using a Jest matcher to simplify expectations

Wouldn't it be great if we could write this? Check it out:

expect(submitSpy).toHaveBeenCalled();

This is more descriptive than using a toBeDefined() argument on the matcher. It also encapsulates the notion that if receivedArguments hasn't been set, then it hasn't been called.

Let's write our first Jest matcher.

We'll spike this code—in other words, not write tests. That's for two reasons: for brevity, and because soon we'll replace this with Jest's own built-in spy functionality. However, I'd encourage you to write tests around all matchers that you use in your code. All matchers and the test "helper" code that you write should be fully test-driven. Just like fakes, they contain logic and need ...