Writing a failing test

What exactly is a test? We'll discover that by writing one. In your project directory, type the following commands:

mkdir test
touch test/Appointment.test.js

Open the test/Appointment.test.js file in your favorite editor or IDE and enter the following:

describe('Appointment', () => {
});

The describe function defines a test suite, which is simply a set of tests with a given name. The first argument is the name (or description) of the unit you are testing. It could be a React component, a function, or a module. The second argument is a function inside of which you define your tests.

For React components, it's good practice to give your describe blocks the same name as the component itself.

You should run this code right now in the Jest test runner. It will give us valuable information about what to do next. You might think that running tests now is pointless, since we haven't even written a test yet, but with TDD, it's normal to run your test runner at every opportunity.

On the command line, run the npm test command:

> appointments@1.0.0 test /home/daniel/work/react-tdd/ch1
> jest

FAIL test/Appointment.test.js
● Test suite failed to run

Your test suite must contain at least one test.

  at node_modules/jest/node_modules/jest-cli/build/TestScheduler.js:225:24

Test Suites: 1 failed, 1 total
Tests: 0 total
Snapshots: 0 total
Time: 0.917s
Ran all test suites.
npm ERR! Test failed. See above for more details.

You can see Jest helpfully tells us Your test suite must contain at least one test. Test-driven developers rely heavily on listening to the test runner and what it tells us. It usually tells them exactly what to do next. In this case, it's telling us to create a test. So, let's do that.