使用 Jest.js 模拟复杂模块

Question

This is what the module I want to mock looks like:

class TheModule {
  constructor() {
    this.subClass = new SubClass();
  }
}

class SubClass {
  constructor() {
    this.someMethod = () => 'Did Something great'; 
  }
}

module.exports = TheModule;

This is TheModule usage I want to test:

const TheModule = require('./TheModule');

const method = () => {
  const theModule = new TheModule();
  return theModule.subClass.someMethod();
}

module.exports = method;

This is my test:

describe('method test', () => {
  
  it('should return "Test pass"', () => {
    jest.doMock('./TheModule');
    const theModuleMock = require('./TheModule');
    const method = require('./TheModuleUsage');
    const mock = () => 'Test pass';
    theModuleMock.subClass.someMethod.mockImplementation(() => mock);
    expect(method()).toEqual('Test pass');
  });
  
});

When I run this test I get TypeError: Cannot read property 'someMethod' of undefined

Is it possible to mock this module without changing TheModule implementation?

Answer 1

If you will export SubClass you can mock it without changing TheModule but in your case you should mock SubClass property in TheModule explicitly with factory for example:

describe('method test', () => {

  it('should return "Test pass"', () => {
    let mockedSomeMethod = jest.fn().mockImplementation(() => 'Test pass');
    jest.doMock('./TheModule', () => {
      // mock constructor
      return jest.fn().mockImplementation(() => {
        return { subClass: { someMethod: mockedSomeMethod  } }
      });
    });
    const method = require('./TheModuleUsage');
    expect(method()).toEqual('Test pass');
  });
});