Jest Mock Behaviour with mockReturnValueOnce

A Jest mock function can be set to return a specific value for all calls, or just once.

Note that if we define a return value with mockReturnValueOnce, the mock function will return undefined for all subsequent calls.
It will not throw an error and may go unnoticed, causing undesirable behaviour.

If a Jest mock returns undefined incorrectly check to make sure how it was defined and how many times it was called.

The code below shows the different behaviours:

describe('mock once tests', () => {
  test('many returns', () => {
    const f = jest.fn()
    f.mockReturnValueOnce('result')

    const result1 = f()
    console.log(result1)

    const result2 = f()
    console.log(result2)

    const result3 = f()
    console.log(result3)
  })

  test('one return', () => {
    const f = jest.fn()
    f.mockReturnValue('result')

    const result1 = f()
    console.log(result1)

    const result2 = f()
    console.log(result2)

    const result3 = f()
    console.log(result3)
  })
})

Output:

result
undefined
undefined

result
result
result