Impossible to test stdout

[melnikaite]

🐛 Bug Report

Mocking process.stdout.write doesn't work like expected

To Reproduce

describe('logger', () => {
    beforeAll(() => {
        jest.spyOn(process.stdout, 'write').mockImplementation(function () { return true; });
    });

    it('should log single string argument', () => {
        console.log('message');
        expect(process.stdout.write).toHaveBeenLastCalledWith('message');
    });
});

Expected: "message"
Received: " console.log
message

  at Object.<anonymous> (logger.test.js:11:17)·

", [Function anonymous]

When I run jest against multiple files jest logger.test.js another.test.js (event if another.test.js) is blank I see another error that means mocking doesn't work at all

FAIL logger.test.js
● Console

console.log
  message

  at Object.<anonymous> (logger.test.js:11:17)

● logger › should log single string argument

expect(jest.fn()).toHaveBeenLastCalledWith(...expected)

Expected: "message"

Number of calls: 0

  10 |     it('should log single string argument', () => {
  11 |         console.log('message');
> 12 |         expect(process.stdout.write).toHaveBeenLastCalledWith('message');
     |                                      ^
  13 |     });
  14 | });
  15 | 

  at Object.<anonymous> (logger.test.js:12:38)

Expected behavior

test pass

envinfo

  System:
    OS: macOS 10.15.4
    CPU: (12) x64 Intel(R) Core(TM) i7-8750H CPU @ 2.20GHz
  Binaries:
    Node: 12.13.1 - ~/.nvm/versions/node/v12.13.1/bin/node
    npm: 6.14.5 - ~/.nvm/versions/node/v12.13.1/bin/npm
  npmPackages:
    jest: ^26.0.0 => 26.0.0 

[jeysal]

This is because of our custom Console implementation. @SimenB ideas on how to help with this use case?

[SimenB]

I'd recommend just mocking console.log instead, and not rely on the implementation detail which is that node's implementation just call stdout.write(string + linefeed)

[jeysal]

Yeah I thought of that as well, implementers of Console objects have quite a lot of freedom. There can be an argument that we try to match Node's behavior as much as possible with things like assertions, require, ... but in this case the UX our console provides is definitely worth it IMO, and I can't think of a better workaround than Simen for this particular use case.

[melnikaite]

I simplified example for this report, but in fact I use winston
So I ended up with jest.spyOn(console._stdout, 'write')

[jeysal]

In that case mocking winston itself would be best I'd say

[SimenB]

I'm very much for making console call through to stdout at some point, PR welcome 😃 But we'd want to mock stdout first, I really like that feature of Jest

[github-actions]

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