Sinon mock class

Question

I have a class:

export default class A {
  data: string
  constructor(data?: any) {
    if (data !== undefined) {
      this.data = data.stingValue
    }
  }
}

Then I have another class which is using A constructor inside a public method:

export default class B {
  public doSomething(data: any) {
    const a = new A(data)
    dependecy.doAnotherThing(a)
  }
}

And test:

it(('shoud doSomething') => {
  const doAnotherThingStub = stub(B.prototype, 'doAnotherThing')
  //this part does not work, just an example of what I would like to achieve
  const doAnotherThingStub = stub(A.prototype, 'constructor').returns({dataReturendFromAConstructorStub: true})
  // end of this part
  const b = new B()
  b.doSomething({})
  expect(doAnotherThingStub.calledWith({dataReturendFromAConstructorStub: true})).to.be.true
})

And my goal is to stub class A constructor. I have separate tests for class A and I do not want to test it again. I need something like stub(A.prototype,'constructor'). I have tried using proxyquire and stubs, but I was not able to inject fake constructor, either real constructor is being called or I am getting something like: A_1.default is not a constructor. Previously I had cases where I need to stub a class which I am calling directly within the test case or stub a method of the class and these are quite straight forward. But I am struggling with this case.

What would be the correct way of mocking the A?

Answer 1

Here is the unit test solution using proxyquire and sinon:

a.ts:

export default class A {
  private data!: string;
  constructor(data?: any) {
    if (data !== undefined) {
      this.data = data.stingValue;
    }
  }
}

b.ts:

import A from './a';

export default class B {
  public doSomething(data: any) {
    const a = new A(data);
  }
}

b.test.ts:

import sinon from 'sinon';
import proxyquire from 'proxyquire';

describe('60152281', () => {
  it('should do something', () => {
    const aStub = sinon.stub();
    const B = proxyquire('./b', {
      './a': {
        default: aStub,
      },
    }).default;
    const b = new B();
    b.doSomething({});
    sinon.assert.calledWithExactly(aStub, {});
  });
});

Unit test results with coverage report:

  60152281
    ✓ should do something (1908ms)


  1 passing (2s)

----------|---------|----------|---------|---------|-------------------
File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
----------|---------|----------|---------|---------|-------------------
All files |   66.67 |        0 |      50 |   66.67 |                   
 a.ts     |   33.33 |        0 |       0 |   33.33 | 4,5               
 b.ts     |     100 |      100 |     100 |     100 |                   
----------|---------|----------|---------|---------|-------------------