Flow: Mocking functions and Flow types

[Nasafato]

So I'm using create-react-app, updated to the latest version of Jest.

I've added Flow annotations to my unit tests, and everything works in terms of type-checking except for when I try to mock module functions.

For example:

describe('dropdown actions', () => {
  test('create close dropdown successfully', () => {
    Date.now = jest.fn().mockReturnValueOnce(1000) // this has a Flow error
    expect(closeDropdown()).toEqual({
      type: 'CLOSE_DROPDOWN',
      closedAt: 1000,
    });
  });
});

Gives me:

 15:     Date.now = jest.fn().mockReturnValueOnce(1000)
              ^^^ property `now`. Covariant property `now` incompatible with contravariant use in
 15:     Date.now = jest.fn().mockReturnValueOnce(1000)
         ^^^^^^^^ assignment of property `now`

I understand why Flow throws this error: Date.now is read only, so now is covariant (it's a static method). That's why Flow is throwing the error - I'm trying to write to the object property.

But Facebook's own documentation for mocking something like Date.now does the same thing I do:

Date.now = jest.fn(() => 1482363367071);

So I guess this something I need to fix by using Jest in a more clever way? Is there a way in Flow to fix this, or make it okay to do this sort of mocking?

[nmn]

Date.now is supposed to be read-only.

The only way around it that I can think of is to force it for testing.

(Date: any).now = ...

[Nasafato]

I see - thanks! I'll go with that for now.

[benoitgrelard]

Hey, I'm having the same issue.

Casting as suggested by @nmn works but I was wondering if there was a way to make Flow aware of the jest mock? Ideally that's what one would be able to do...

[nmn]

@artisologic You could define your own module just for testing. Then you could import them in Flow comments so Flow uses those types.

So for example, you could define a file like this for your own Date type:

declare module.exports: {
  now: Function,
  ...
}

Then in your test you could:

/*::
const Date = require('path-to-fake-date');
*/

Now, Flow would use the types from the module you defined even though it won't actually affect your code.

[sebastienbarre]

tangentially, how are you guys not getting all the warning about jest globals?

src/api/__tests__/timers.test.js:5
  5: jest.mock('api/request');
     ^^^^ identifier `jest`. Could not resolve name

src/api/__tests__/timers.test.js:7
  7: describe('fetchTimers', () => {
     ^^^^^^^^ identifier `describe`. Could not resolve name

src/api/__tests__/timers.test.js:8
  8:   it('returns a few timers (promise)', () => fetchTimers().then((timers) => {
       ^^ identifier `it`. Could not resolve name

I tried:

$ flow-typed install jest
• Found [email protected] installed. Installing libdefs compatible with this version of Flow...
ERROR: Please specify an npm package name of the format '[email protected]'

So I tried:

$ flow-typed install jest@19
• Found [email protected] installed. Installing libdefs compatible with this version of Flow...
• rebasing flow-typed cache...done.
• Installing 1 libdefs...
  • jest_v19.x.x.js
    └> ./flow-typed/npm/jest_v19.x.x.js

thinking that create-react-app uses v19.
But I'm getting the same errors...
Thanks