Can't mock localstorage method with jest.fn

🐛 Bug Report

It is impossible to mock a localStorage method

To Reproduce

Using the following code:

it( 'setItem mock fn', () => {
	const mockFn = jest.fn( localStorage.setItem );
	localStorage.setItem = mockFn;
	localStorage.setItem( 'foo', 'bar' );
	expect( mockFn ).toHaveBeenCalledTimes( 1 );
} );

I have the following error: Expected mock function to have been called one time, but it was called zero times.

Expected behavior

No error should be thrown, since setItem is called during the test.

Link to repl or repo (highly encouraged)

Repo

Run npx envinfo --preset jest

  System:
    OS: Linux 4.15 Pop!_OS 18.04 LTS
    CPU: x64 Intel(R) Core(TM) i7-8700 CPU @ 3.20GHz
  Binaries:
    Node: 10.9.0 - /usr/local/bin/node
    npm: 6.2.0 - /usr/local/bin/npm
  npmPackages:
    jest: ^23.5.0 => 23.5.0

Comments

I suppose this is related to an error I saw earlier, making me think that localStorage and sessionStorage are proxies.

[GerkinDev]

Brilliant! For those with the error, simply use spy: const spy = jest.spyOn(Storage.prototype, 'setItem');

[stevehanson]

For those wanting to mock localstorage and not simply spy on it, this worked for me:

Storage.prototype.getItem = jest.fn(() => 'bla');