Feature Request: Expose return values of calls to mock functions

[UselessPickles]

Do you want to request a feature or report a bug?
Feature

What is the current behavior?
There is currently no way to access the return values that were returned by calls to mock functions.

What is the expected behavior?
When using jest.mock() to simply wrap a method for the purpose of "spying" on calls to the method (check if it's called, inspect the params it was called with, etc), it would also be very useful to have access to the list of return values that have been returned by calls to the mock function. This would be equivalent to sinon's spy.returnValues: http://sinonjs.org/releases/v2.0.0/spies/

The basic design of this would involve adding "returnValues" to MockFunctionState as such:

type MockFunctionState = {
  instances: Array<any>,
  calls: Array<Array<any>>,
  returnValues: Array<any>,
  timestamps: Array<number>,
};

And updating _makeComponent to push the final returned value onto the new returnValues array.

(see https://github.com/facebook/jest/blob/master/packages/jest-mock/src/index.js)

If there are no major "gotchas" to this plan, then I would volunteer to create a PR with these changes.

Please provide your exact Jest configuration and mention your Jest, node,
yarn/npm version and operating system.
Irrelevant

[SimenB]

I think that would be a nice feature. PR welcome! 🙂

[UselessPickles]

I'll work on this in the next few days. I suspect the code change itself will be fairly easy, and the bulk of the effort will be updating unit tests to thoroughly verify it.

[UselessPickles]

Can anyone help me out with this error when running yarn test?

D:\DevProjects\github\jest>yarn test
yarn run v1.5.1
$ yarn typecheck && yarn lint && yarn jest
$ flow check --include-warnings
Found 0 errors
$ eslint . --cache --ext js,md

D:\DevProjects\github\jest\integration-tests\transform-linked-modules\ignored\symlink.js
  1:1  error  Parsing error: Unexpected token

> 1 | ../package/index.js
    | ^

✖ 1 problem (1 error, 0 warnings)

It seems completely unrelated to the changes I'm making. Here's the branch with my changes: https://github.com/UselessPickles/jest/tree/mock_return_values

[UselessPickles]

Hmmm... there's also several unit tests failing in master. I guess the build is generally a bit broken even before I made any changes.

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