Test if an array is empty or contains a certain object with

Question

I want to test if an array is empty or contains objects of a certain structure. In pseudo code it could be something like this:

expect([])
  .toHaveLength(0)
  .or
  .arrayContaining(
    expect.toMatchObject(
     { foo: expect.any(String) }
    )
  ) => true

expect([{foo: 'bar'}])
  .toHaveLength(0)
  .or
  .arrayContaining(
    expect.toMatchObject(
     { foo: expect.any(String) }
    )
  ) => true

expect([1])
  .toHaveLength(0)
  .or
  .arrayContaining(
    expect.toMatchObject(
      { foo: expect.any(String) }
    )
  ) => false

Maybe I'm getting it wrong on the how things work in Jest, but to me my problem looks like an or-question.

Mitch Lillie · Accepted Answer · 2018-08-30 00:42:36Z

4

The best way to encapsulate a logical or in your test suite would be to just exclude the particular case inside of the test itself. For example:

it('has an array of objects', () => {
  if (arr.length > 0) {
    expect(arr).toBe(expect.arrayContaining(expect.toMatchObject({ foo: expect.any(String) })));
  }
});

However, I think a more unit test-friendly approach would be to separate your concerns. One test for when there is an empty array, and one test for when the foo property is set to 'bar', etc.

answered Aug 30, 2018 at 0:42

Mitch Lillie

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2 Comments

Maximilian Lindsey Over a year ago

Thanks, I also found this article that, in combination with what you wrote, was very helpful medium.com/@andrei.pfeiffer/…

Mitch Lillie Over a year ago

Yes, I think a custom matcher would be a great solution if you were finding yourself testing this case more often. Still, I wonder if you're writing one test case where you should be writing two

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