Testing instances in a function that depends on instanceof (JEST)

Question

I am running into an issue while writing unit test to test a function that has instance check. Business logic:

let status = new B();
const testFunction = () => {
    if (status instanceof A) {
      console.log('inside A')
    } else if (status instanceof B) {
      console.log('inside B')
    } else {
      console.log('No matchers')
    }

}

Unit Test:

describe('test', ()=>{
  
  it("should test b", ()=>{
    let status = new A()
    expect(status).toBeInstanceOf(A) // passes
    expect(logSpy).toHaveBeenCalledWith('inside A') // fails and it expects 'inside B'
  })
})

One reason I feel the test is failing is because in the original file, status is return an instance of class B, so it matches else if and not if. I am not sure how to test this code without change status? Is there a way that I am missing out?

EDIT: Here the assumption is that the testFunction() is already invoked in unit tests

Answer 1

You can pass the status as parameter of your function;

const testFunction = (status) => {
  if (status instanceof A) {
    console.log('inside A')
  } else if (status instanceof B) {
    console.log('inside B')
  } else {
    console.log('No matchers')
  }
}

Then testFunction() should be called with the status that you need to test;

const status = new A();
testFunction(status);

The problem with your test is that the function that your are testing has its own scope, the other solution is to mock the variable, but could be most confused