toHaveBeenCalledWith indifferent to parameters that have `undefined` or `null` as a value

🐛 Bug Report

When mocking a function which takes parameters, if one of the parameter’s value is undefined, toHaveBeenCalledWith can be called with or without that same parameter as an expected parameter, and the assertion will pass.

To Reproduce

Code:

# call-with-fn.js

function callFnWith(a, fn) {
  const result = fn({ arg: a });
  return result;
}

module.exports = callFnWith;

Test:

# call-with-fn-test.js

const callFnWith = require('./call-fn-with');
describe('undefined arg', () => {
  const fn = jest.fn(() => ({ arg: 3, result: undefined }));
  callFnWith(3, fn);
  it('should include an undefined arg', () => {
    // I would have assumed I needed to include "result: undefined" below:
    expect(fn).toHaveBeenCalledWith({ arg: 3 }); 
  });
});

The test passes with both variants of this assertion:

    expect(fn).toHaveBeenCalledWith({ arg: 3 }); 
    expect(fn).toHaveBeenCalledWith({ arg: 3, result: undefined }); 

Expected behavior

I would have expected the assertion to fail with the first variant above.

Link to repl or repo (highly encouraged)

https://repl.it/repls/PaltryNuttyEmacs

Issues without a reproduction link are likely to stall.

Run npx envinfo --preset jest

Paste the results here:

System:
    OS: macOS 10.14.4
    CPU: (8) x64 Intel(R) Core(TM) i7-4850HQ CPU @ 2.30GHz
  Binaries:
    Node: 10.15.1 - ~/.nvm/versions/node/v10.15.1/bin/node
    npm: 6.8.0 - ~/.nvm/versions/node/v10.15.1/bin/npm
  npmPackages:
    jest: ^24.6.0 => 24.6.0 

[pedrottimark]

@twelve17 in addition to what Tim said in preceding comment, study your example code to see:

• fn is called with { arg: 3 }
• fn returns { arg: 3, result: undefined }

For more information:

• https://jestjs.io/docs/en/mock-functions
• https://jestjs.io/docs/en/mock-function-api

If you make some assumptions about number of calls, you can write specific assertions:

describe('undefined arg', () => {
  const fn = jest.fn(() => ({ arg: 3, result: undefined })); // see results
  callFnWith(3, fn);

  test('calls', () => {
    expect(fn).toHaveBeenCalledWith({ arg: 3 });
    expect(fn).toHaveBeenCalledWith({ arg: 3, result: undefined }); // confusing

    console.log(fn.mock.calls);
    // [ [ { arg: 3 } ] ]

    expect(fn.mock.calls[0][0]).toStrictEqual({ arg: 3 });
    expect(fn.mock.calls[0][0]).not.toStrictEqual({ arg: 3, result: undefined });
  });
  test('results', () => {
    expect(fn).toHaveReturnedWith({ arg: 3 }); // confusing
    expect(fn).toHaveReturnedWith({ arg: 3, result: undefined });

    console.log(fn.mock.results);
    // [ { type: 'return', value: { arg: 3, result: undefined } } ]

    expect(fn.mock.results[0].value).not.toStrictEqual({ arg: 3 });
    expect(fn.mock.results[0].value).toStrictEqual({ arg: 3, result: undefined });
  });
});

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