Mastering React Test-Driven Development

Making spies reusable

We know that we have another bunch of tests within CustomerForm, and we have similar tests for AppointmentForm. How can we reuse what we've built in this one test across everything else? We can create a generalized spy function that can be used any time we want spy functionality.

Using hand-rolled test doubles is perfectly acceptable; you shouldn't feel the need to always generalize as we're about to do.

Let's start by defining a function that can stand in for any single-argument function, such as the event handlers we would pass to the onSubmit form prop:

1. Define the following function at the top of test/CustomerForm.test.js:

const singleArgumentSpy = () => {  let receivedArgument;  return { fn: arg => (receivedArgument ...