Jest: the second argument of jest.mock must be a function

[01binary]

What is the current behavior?

jest.mock(moduleName, func) throws the error, the second argument of jest.mock must be a function if func is defined as:

function func() {
   return 'Hello';
}

...but works if the func is defined inline as an arrow function:
jest.mock(moduleName, () => 'Hello')

In both cases moduleName is a string like 'fs'

What is the expected behavior?

Either of:

• Change error message to say the second argument of jest.mock must be an arrow function
• Make jest.mock accept a function that's not an arrow function

This is in a bare-bones Node app that doesn't use Babel and has index.js that prints a message to console.

[cpojer]

It accepts functions, it just doesn't accept references. If you'd like, you could send a PR to babel-plugin-jest-hoist to adjust the error message to say must be an inline function. What do you think?

[nicolasiensen]

@cpojer is there a good reason why mock it doesn't accept function reference?

[cpojer]

Yes, because your mock may be required before references are fully initialized. It's easier to see jest.mock calls as fully isolated, and the function as an isolated module scope.

Consider a module Banana that depends on Kiwi, which is mocked:

import Banana from './Banana';
jest.mock('Kiwi', () => name);
const name = ''I am a pear';

When Banana requires Kiwi, we know Kiwi is mocked. We run the function and then it throws saying name does not exist/is not initialized, simply because it is sequentially executed:

• require Banana
• Banana requires Kiwi
• Kiwi is mocked, and runs the mock function.
• The mock function would like to return name.
• name isn't initialized yet

This is why you cannot reference anything from the outer scope. Hope that makes sense.

[cpojer]

Thanks! See https://github.com/facebook/jest/pull/4593

[mrlubos]

Hi, is there a way to automatically call mock on every test suite? I ran into this (now updated) error message when trying to create a mock module for i18n and call it as jest.mock('react-i18next', fn);.

I know this function will be required in multiple modules so I am trying to avoid writing this fix in every test.

jest.mock('react-i18next', () => ({
  // this mock makes sure any components using the translate HoC receive the t function as a prop
  translate: () => Component => props => <Component t={() => ''} {...props} />,
}));

[SimenB]

@lmenus add your mock to a file called react-i18next in __mocks__. See https://facebook.github.io/jest/docs/en/manual-mocks.html

[mrlubos]

@SimenB Ah, missed that, thank you! This certainly reduces it to a mere jest.mock('react-i18next'); now!