How to use jest to mock a function just for some specified arguments, but uses its original logic for any other arguments?

2021-03-08 17:47 Freewind imported from Stackoverflow

Say I have a function:

function complexCompute(num: number): string {
   switch(num) {
      case 1: return '...something...';
      case 2: return '...something...';
      case 3: return '...something...';
      // more cases
}

It is used many times in the code I want to test, but I want to mock it like this:

when I pass the argument num is 1, complexCompute returns my mocked string my-mock-string1
when I pass any other argument, it uses its original logic

I can't find a way to do it, since if I mocked the module:

jest.mock('./complexCompute')

The mocked complexCompute doesn't have original logic. I have to define the mock value for argument 1, and also rewrite the whole logic for other arguments.

Is there any way to do it?

1 answer

answered 2021-03-09 06:47 Teneff

To access the actual implementation for a mocked module you can use jest.requireActual(moduleName).

Here are two examples:

Mocking the entire module

jest.mock('./complexCompute', () => {
  const actual = jest.requireActual('./complexCompute').default;

  return {
    default: (arg: number): string => arg === 1 ? 'my-mock-string1' : actual(arg)
  }
})

Using auto-mock with .mockImplementation

import complexCompute from './complexCompute';

jest.mock('./complexCompute')

complexCompute.mockImplementation((arg) => {
  return arg === 1
    ? 'my-mock-string1'
    : jest.requireActual('./complexCompute').default(arg)
});

working TypeScript example

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How to use jest to mock a function just for some specified arguments, but uses its original logic for any other arguments?

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