Flow: Mutation of module exports

Asked May 09 '17 at 18:53

Active May 09 '17 at 18:53

Viewed 397 times

I have a module which exports a function:

// module.js

export function doSomething() {
  // ...
}

I need to mock this function in tests. According to this answer, I can do the following:

// module.test.js

import * as module from './module';

it('does something', () => {
    module.doSomething = jest.fn(() => console.log('Something was done'))

    // test
})

This code works (the function implementation is being replaced), but Flow is unhappy: Error:(101, 3) Flow: assignment of property 'doSomething'. Mutation not allowed on exports of "./module"

I can't seem to find any information about this particular error (both Google and Stack Overflow search give me exactly 0 results).

Are these complaints justified or can I just ignore them?

If it's the latter, what's the way to explain to Flow that this is nothing to worry about (other than using // $FlowExpectedError in every case)?

edited May 23 '17 at 12:18

Community

asked May 09 '17 at 18:53

Andrii Chernenko

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1

As the answer you linked to says, `solve this by using a hack`. You're now seeing why this is a hack. Reassigning an export from outside the module is not allowed by ES6. It does work with Babel 6 though. – loganfsmyth May 09 '17 at 19:43
^ This. Btw, Jest provides a way to mock CJS modules (which are likely used under the hood for ES6 module implementation). – Estus Flask May 09 '17 at 20:09
okay, fair enough. I guess I'll have to find another way. – Andrii Chernenko May 10 '17 at 08:20
Did you find any solution to this problem? – Alexandre Nov 29 '17 at 14:50
1

@Alexandre no, I just accepted the fact that Flow is not quite mature yet and moved on. – Andrii Chernenko Dec 04 '17 at 08:51

Flow: Mutation of module exports

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