What is the easiest way to test event listeners in jest?

Question

I'm trying to test the below function

 getVal(process) {
    test.on('data', async data => {
    try {
       for (const val of data) {
       await process(val);
       console.log('processed')
    }} catch (e) {}
    });
    test.on('error', err => {
       console.log('error', err)
     });
    }

process(payload) {
return new Promise(resolve=>{.....})
};

Jest test: // in beforeEach

mockData =[an array containing 10 values] 
    onSpy = jest
          .fn()
          .mockImplementationOnce(async (data, callback) => {
            callback(mockData);
          })
          .mockImplementationOnce((error, callback) => {
            callback(mockErr);
          });

 it('should trigger callback once per message', async () => {
    await xyz.getVal(process);
    await expect(process).toHaveBeenCalledTimes(10);
 });

I expect the process() to be called 10 times since the data is sent 10 times. However it's called only 2 times when I assert but when I put a console.log in the function itself and run the test, that gets called 10 times.

I'm not sure what's wrong. Appreciate any help.

Solution

The issue is that calling await on xyz.getVal doesn't actually do anything since getVal is a synchronous function that just sets up the event listeners...

...so the asynchronous events haven't finished processing by the time the expect runs and fails.

---

It looks like you've already got a spy on test.on.

Instead of mocking its implementation, just use it to get the callback function.

Then you can call and await the callback function directly:

  // in beforeEach
  mockData = [an array containing 10 values]
  onSpy = jest.fn();

it('should trigger callback once per message', async () => {
  xyz.getVal(process);
  const callback = onSpy.mock.calls[0][1];  // <= get the callback (second argument of the first call to test.on)
  await callback(mockData);  // <= call and await the callback directly
  expect(process).toHaveBeenCalledTimes(10);  // Success!
});