cannot call mockImplementationOnce on empty function

[bschelling]

Affected URL(s)

https://nodejs.org/en/learn/test-runner/mocking

Description of the problem

The example given "Modules + units" uses a call to mock.fn() without any parameters, causing " barMock.mockImplementationOnce" to fail later on.

I am not sure if the example is only for demonstration purposes but I tried to run it :)

const barMock = mock.fn();

Later on it calls:

 barMock.mockImplementationOnce(function bar_mock() {
      /* … */
    });

Since the barMock function did not receive a function parameter above, the call to barMock.mockImplementationOnce fails.

[cjihrig]

I'm not sure if that example is supposed to be runnable or not, but I'm unable to reproduce this with the following code:

const { test, mock } = require('node:test');

test(() => {
  const fn = mock.fn();

  fn();
  fn();
  fn.mock.mockImplementationOnce(() => {
    console.log('here');
  });
  fn();
  fn();
});

Are you able to provide a different minimal example that fails? Or could you at least include the actual error you're seeing?

EDIT: Oh, it looks like that example is doing barMock.mockImplementationOnce() instead of barMock.mock.mockImplementationOnce().

[cjihrig]

Docs fixed in nodejs/nodejs.org#7779.