javascript - Passing parameters to jest function - Stack Overflow

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I have this function in my child ponent:

<div> {handTotal('dealersHand').total} </div>

This has been passed down from above

however when I run jest it says Cannot read property 'total' of undefined

when i console.log(handTotal('dealersHand') it is logging the right thing and the function works so I know it's doing the correct thing

I've stubbed it out in jest like so:

const handTotalStub = jest.fn()
  beforeEach(() => {
    wrapper = mount(<Dealer
      dealersHand={dealersHandStub}
      containsAce={containsAceStub}
      handTotal={handTotalStub}
    />);
  })

How do I pass the parameter into this function so that jest understands what it is?

can add more code/explanation if doesn't make sense

stubbing:

  const handTotalStub = jest.fn().mockReturnValue('dealersHand')

test:

  it('expects dealers hand to equal', () => {
    expect(handTotalStub('dealersHand').total).toEqual(1);
  });

I have this function in my child ponent:

<div> {handTotal('dealersHand').total} </div>

This has been passed down from above

however when I run jest it says Cannot read property 'total' of undefined

when i console.log(handTotal('dealersHand') it is logging the right thing and the function works so I know it's doing the correct thing

I've stubbed it out in jest like so:

const handTotalStub = jest.fn()
  beforeEach(() => {
    wrapper = mount(<Dealer
      dealersHand={dealersHandStub}
      containsAce={containsAceStub}
      handTotal={handTotalStub}
    />);
  })

How do I pass the parameter into this function so that jest understands what it is?

can add more code/explanation if doesn't make sense

stubbing:

  const handTotalStub = jest.fn().mockReturnValue('dealersHand')

test:

  it('expects dealers hand to equal', () => {
    expect(handTotalStub('dealersHand').total).toEqual(1);
  });

• javascript
• reactjs
• mocking
• jestjs

Share Improve this question Follow edited Apr 8, 2018 at 7:36 The Walrus asked Apr 8, 2018 at 7:08 The WalrusThe Walrus 1,20877 gold badges3131 silver badges4949 bronze badges

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1 Answer 1

Sorted by: Reset to default 3

Your jest spy for handleTotalStub does not return anything, so it therefore returns undefined. When your ponent tries to call handTotal('dealersHand').total it is therefore calling undefined.total, because handTotal(...) is not defined.

Update your spy to return something (anything) by changing

handTotalStub = jest.fn();

to

handTotalStub = jest.fn().mockReturnValue(SOME_VALUE);

or

handTotalStub = jest.fn().mockImplementation(() => SOME_VALUE);

(where SOME_VALUE is any value that you can mock out to act as what the ponent expects to be there)

EDIT -- Ok, so you're misunderstanding what mockReturnValue does. You don't have to mock the parameters being passed in to that method. Because your ponent is already passing in that string. BUT, the actual handTotal method is never going to be called (this is a good thing, because we're not testing how handTotal works, we're testing how the ponent works).

So, whatever handTotal would normally return is what you want to put in mockReturnValue(). So if handTotal returns an object like {total: 1}, then you would say mockReturnValue({total: 1})

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