Jest: How to mock one specific method of a class

Question

Let\'s suppose I have the following class:

export default class Person {
    constructor(first, last) {
        this.first = first;
        this.last = last         


        

Answer 1

Not really answer the question, but I want to show a use case where you want to mock a dependent class to verify another class.

For example: Foo depends on Bar. Internally Foo created an instance of Bar. You want to mock Bar for testing Foo.

Bar class

class Bar {
  public runBar(): string {
    return 'Real bar';
  }
}

export default Bar;

Foo class

import Bar from './Bar';

class Foo {
  private bar: Bar;

  constructor() {
    this.bar = new Bar();
  }

  public runFoo(): string {
    return 'real foo : ' + this.bar.runBar();
  }
}

export default Foo;

The test:

import Foo from './Foo';
import Bar from './Bar';

jest.mock('./Bar');

describe('Foo', () => {
  it('should return correct foo', () => {
    // As Bar is already mocked,
    // we just need to cast it to jest.Mock (for TypeScript) and mock whatever you want
    (Bar.prototype.runBar as jest.Mock).mockReturnValue('Mocked bar');
    const foo = new Foo();
    expect(foo.runFoo()).toBe('real foo : Mocked bar');
  });
});

See also jest.requireActual(moduleName)