expect(value)

The expect function is used every time you want to test a value. You will rarely call expect by itself. Instead, you will use expect along with a "matcher" function to assert something about a value.

It's easier to understand this with an example. Let's say you have a method bestLaCroixFlavor() which is supposed to return the string 'grapefruit'. Here's how you would test that:

test('the best flavor is grapefruit', () => {
  expect(bestLaCroixFlavor()).toBe('grapefruit');
});

In this case, toBe is the matcher function. There are a lot of different matcher functions, documented below, to help you test different things.

The argument to expect should be the value that your code produces, and any argument to the matcher should be the correct value. If you mix them up, your tests will still work, but the error messages on failing tests will look strange.