jest-create-mock-instance

Description

This library uses jest-mock internal package to automatically create & instantiate mock for given class/function constructor.

I've been always fan of using sinon.createStubInstance() in my tests. Unfortunately, Jest doesn't expose similar API to create new mocked instance for given class constructor. Using jest.fn() or jest's module mocks is not very convienment for various cases, take an example of these classes and unit test:

food.js

export class Food {
    consume() {}
}

cat.js

export class Cat {
    constructor(food) {
        this.food = food;
    }
    eat() {
        this.food.consume();
    }
}

and test:

cat.spec.js

import { Food } from "../food";
import { Cat } from "../cat";

let cat;
let food;
beforeEach(() => {
    food = new Food();
    cat = new Cat(food);
});

it("Must eat", () => {
    cat.eat();
    expect(food.consume).toBeCalled();
});

By using jest module mocks you must mock your module before and instantiate Food directly:

jest.mock("../food");
import { Food } from "../food";

beforeEach(() => {
    food = new Food();
})

This is very ugly when have many modules to mock (and you don't want automock by default) and when using typescript:

jest.mock("../food1");
jest.mock("../food2");
jest.mock("../food3");
import { Food1 } from "../food1";
import { Food2 } from "../food2";
import { Food3 } from "../food3";

let food1: jest.Mocked<Food1>;
let food2: jest.Mocked<Food2>;
let food3: jest.Mocked<Food3>;
beforeEach(() => {
    // Food constructors may have other required parameters so often necessary cast them to any
    food1 = new (Food1 as any)() as any;
    food2 = new (Food2 as any)() as any;
    food3 = new (Food3 as any)() as any;
});

Instead you can just write:

import createMockInstance from "jest-create-mock-instance";
import { Food } from "../food";

let food: jest.Mocked<Food>;
beforeEach(() => {
    food = createMockInstance(Food);
});

Install

npm install jest-create-mock-instance --save-dev

Typescript

createMockInstance returns jest.Mocked<T> object. Don't forget to install @types/jest package.