mock-function

A simple mock function to mock functions.

Inspired by jest.fn.

Installation

Install the package.

# NPM
npm i --save-dev mock-function
# yarn
yarn add --dev mock-function

Import it in your tests.

import fn from 'mock-function';

Usage

import mockFunction from 'mock-function';

const add = (a: number, b: number) => a + b;
const mockedAdd = mockFunction(add);

mockedAdd.hasBeenCalled();
// ↵ false

mockedAdd.hasBeenCalledWith(21, 21);
// ↵ false

mockedAdd.hasBeenCalledTimes;
// ↵ 0
mockedAdd.calls;
// ↵ []

mockedAdd(21, 21);
// ↵ 42
mockedAdd(9000, 1);
// ↵ 9001 (😱 OVER 9000)

mockedAdd.hasBeenCalled();
// ↵ true

mockedAdd.hasBeenCalledWith(2000, 12);
// ↵ false
mockedAdd.hasBeenCalledWith(21, 21);
// ↵ true

mockedAdd.hasBeenCalledTimes;
// ↵ 2
mockedAdd.calls;
// ↵ [[21, 21], [9000, 1]]

Caveats

Mocking is a code smell for tight coupling and as such a surface indication that you might be able to improve your tests and / or underlying code. Think about whether you can isolate your side-effects or non-deterministic functions and make your code more modular. Mocking can be okay, for example in integration tests.

For example, the pure function add in Usage should be tested using plain unit tests that assert the actual and expected output. I chose add to simplify the example, but it is generally a bad use-case for mocking.