expect.assertions(number)

expect.assertions(number) verifies that a certain number of assertions are called during a test. This is often useful when testing asynchronous code, in order to make sure that assertions in a callback actually got called.

For example, let's say that we have a function doAsync that receives two callbacks callback1 and callback2, it will asynchronously call both of them in an unknown order. We can test this with:

  1. test('doAsync calls both callbacks', () => {
  2.   expect.assertions(2);
  3.   function callback1(data) {
  4.     expect(data).toBeTruthy();
  5.   }
  6.   function callback2(data) {
  7.     expect(data).toBeTruthy();
  8.   }
  10.   doAsync(callback1, callback2);
  11. });

The expect.assertions(2) call ensures that both callbacks actually get called.