Use rewire with Jest, to spy on console.log used in a private function

I have a private function in one file, which uses console.log. I want to check that console.log does indeed run, in my Jest test. Therefore to access the private function, I use rewire.

console.log

console.log

I have the following file:

// a.js
function b()
console.log('c');



And I have the following test file, where I use the method suggested here to replace console.log with a Jest mock function, and the method here to ensure the replacement gets made before the rewire:

console.log

rewire

// a.test.js
global.console =
log: jest.fn(),
;

const rewire = require('rewire');
const a = rewire('./a');

test('b', () =>
a.__get__('b')();
expect(global.console.log).toHaveBeenCalled();
);


Yet when I run the test, I get:

● Test suite failed to run

logger must implement log, warn and error methods


And if I use the following test code instead:

// a.test.js
const rewire = require('rewire');
const a = rewire('./a');

a.__set__('console',
log: jest.fn(),
);

test('b', () =>
a.__get__('b')();
expect(global.console.log).toHaveBeenCalled();
);


I get the following error:

expect(jest.fn())[.not].toHaveBeenCalled()

jest.fn() value must be a mock function or spy.
Received:
function: [Function bound log]


Is there any way at all to spy on console.log when used in a private function?

console.log

1 Answer
1

Keep track of the mock you give __set__ and assert on it directly:

__set__

const rewire = require('rewire');
const a = rewire('./a');

const logMock = jest.fn(
(...args) => console.log('logMock called with', ...args)
);

a.__set__('console',
log: logMock,
);

test('b', () =>
a.__get__('b')();
expect(logMock).toHaveBeenCalled();
);


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