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
`js
export class Food {
consume() {}
}

`

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

and test:

cat.spec.js
`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:

`js
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:

`ts
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;
let food2: jest.Mocked;
let food3: jest.Mocked;
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:

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

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

Install

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

Typescript

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