Jest mocking of classes with DI dependencies

Question

Various Jest docs show creation of "automatic" mocks, "manual" mocks, or ES6 class mocks (which instantiate dependencies within the constructor).

But I want to use DI / IOC and inject dependencies into the ctor:

// IBar.ts                                           <--- mock this
export default interface IBar {
  /* ...methods... */
}

// Baz.ts                                            <--- mock this
export default class Baz {
  constructor(spam: Spam, ham: IHam) { /* ... */}
  /* ...other methods... */
}

// Foo.ts                                            <--- test this
export default class Foo {
  constructor(bar: IBar, baz: Baz) { /* ... */}
  /* ...other methods... */
}

So I want to do this in a test:

const barMock = jest.giveMeAMock("../../IBar");  // or jest.giveMeAMock<IBar>();
const bazMock = jest.giveMeAMock("./Baz");       // or jest.giveMeAMock<Baz>();
const foo = new Foo(bar, baz);

expect(foo.something()).toBe(true);

Is this possible with Jest?

(I used some TypeScript syntax above, but it's the same problem for JS/ES6 and TS.)

Answer 1

TypeScript interfaces just get compiled away when the code gets converted to JavaScript...

...but it's definitely possible for a class.

You can auto-mock a module using jest.mock and Jest will keep the API surface of the module the same while replacing the implementation with empty mock functions:

baz.js

export default class Baz {
  doSomething() {
    throw new Error('the actual function throws an error');
  }
}

foo.js

export default class Foo {
  constructor(baz) {
    this.baz = baz;
  }
  doSomething() {
    // ...
    this.baz.doSomething();
    // ...
  }
}

code.test.js

jest.mock('./baz');  // <= auto-mock the module

import Baz from './baz';
import Foo from './foo';

test('Foo', () => {
  const baz = new Baz();  // <= baz is an auto-mocked instance of Baz
  const foo = new Foo(baz);

  foo.doSomething();  // (no error)

  expect(baz.doSomething).toHaveBeenCalled();  // Success!
})