jest-when

npm i --save-dev jest-when

A sugary way to mock return values for specific arguments only.

Basic usage:

import { when } from 'jest-when';

const fn = jest.fn();
when(fn).calledWith(1).mockReturnValue('yay!');

const result = fn(1);
expect(result).toEqual('yay!');

Supports multiple args:

import { when } from 'jest-when';

const fn = jest.fn();
when(fn).calledWith(1, true, 'foo').mockReturnValue('yay!');

const result = fn(1, true, 'foo');
expect(result).toEqual('yay!');

Supports jest matchers:

import { when } from 'jest-when';

const fn = jest.fn();
when(fn).calledWith(
  expect.anything(),
  expect.any(Number),
  expect.arrayContaining(false)
).mockReturnValue('yay!');

const result = fn('whatever', 100, [true, false]);
expect(result).toEqual('yay!');

Supports compound declarations:

import { when } from 'jest-when';

const fn = jest.fn();
when(fn).calledWith(1).mockReturnValue('no');
when(fn).calledWith(2).mockReturnValue('way?');
when(fn).calledWith(3).mockReturnValue('yes');
when(fn).calledWith(4).mockReturnValue('way!');

expect(fn(1)).toEqual('no');
expect(fn(2)).toEqual('way?');
expect(fn(3)).toEqual('yes');
expect(fn(4)).toEqual('way!');
expect(fn(5)).toEqual(undefined);

Assert the args:

Use expectCalledWith instead to run an assertion that the fn was called with the provided args. Your test will fail if the jest mock function is ever called without those exact expectCalledWith params.

Disclaimer: This won't really work very well with compound declarations, because one of them will always fail, and throw an assertion error.

import { when } from 'jest-when';

const fn = jest.fn();
when(fn).expectCalledWith(1).mockReturnValue('x');

fn(2); // Will throw a helpful jest assertion error with args diff