title: Should I use jest.fn() for all mocked functions, even when it's not used?
body: <p>I can create a mock function either with <code>jest.fn(() =&gt; 1)</code> or with <code>() =&gt; 1</code>. Should I do the first even when I don't need it? (eg I'm not using <code>expect().toHaveBeenCalled()</code>)</p>
  
  <pre><code>const foo = ({ helper }) =&gt; helper()
  
  test('example v1', () =&gt; {
    const arg = { 
      helper: jest.fn(() =&gt; 1) 
    }
    expect(foo(arg)).toBe(1)
  })
  
  test('example v2', () =&gt; {
    const arg = { 
      helper: () =&gt; 1
    }
    expect(foo(arg)).toBe(1)
  })
  </code></pre>
  
  <p>Possible benefits of always using <code>jest.fn()</code> include:</p>
  
  <ul>
  <li>documentation—knowing it's supposed to be a mock function</li>
  <li>easy to add <code>expect(arg.helper).toHaveBeenCalled*()</code> later</li>
  </ul>