React testing library with jest : trying to test the existence of a button with toHaveLength fails. Why?

I just started trying out react testing library with jest, going through a tutorial and have the following question:

const App = () => {
  return (
    <form onSubmit={handleSubmit}>
      <div>
        <label>Name</label>
        <input value={name} onChange={(e) => setName(e.target.value)} />
      </div>
      <div>
        <label>Email</label>
        <input value={email} onChange={(e) => setEmail(e.target.value)} />
      </div>
      <button>Add User</button>
    </form>
  );
}

the test should locate the button but shows an error:

test('shows 2 inputs and a button', async () => {
    render(<App />);
    const btn = await screen.findByRole('button');
    expect(btn).toHaveLength(1)
})

this however works:

test('shows 2 inputs and a button', async () => {
    render(<App />);
    const btn = await screen.findByRole('button');
    expect(btn).toBeInTheDocument()
})

Why is the first test failing locating the button with toHaveLength?

It fails because screen.findByRole('button') returns a single element, not an array of elements. The toHaveLength method is used to check the length of an array, so it doesn't work with a single element.

To check the number of buttons in the document, you should use screen.findAllByRole('button'), which returns an array of elements, and then you can use toHaveLength.

test('shows 2 inputs and a button', async () => {
render(<App />);
const btns = await screen.findAllByRole('button');
expect(btns).toHaveLength(1);
});