Add auto-mock support for generators

[steven10172]

Summary

Fixes #5979

Test plan

Integration Tests Included

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[codecov-io]

Codecov Report

Merging #5983 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #5983   +/-   ##
=======================================
  Coverage   64.36%   64.36%           
=======================================
  Files         217      217           
  Lines        8292     8292           
  Branches        3        3           
=======================================
  Hits         5337     5337           
  Misses       2954     2954           
  Partials        1        1

Impacted Files	Coverage Δ
packages/jest-mock/src/index.js	87.34% <100%> (ø)	⬆️

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[mjesun]

LGTM, thanks for the contribution! ❤️

A possible future step might be to make the default mock return an empty Generator (i.e. a Generator instance with next, return, throw, etc); which defaults to a finished one with undefined. This way code expecting a mocked generator would still work. WDYT?

[steven10172]

@mjesun wouldn't that break existing auto-mock behavior and require everyone to always have to mock the generator. Currently as it users would be able to use the generator as is and provide a mock if they please.

[steven10172]

@mjesun Actually you're right. I will explore a fix for that. In the simplest sense I plan on getting this test to pass.

jest.mock('./module');
const mod = require('./module');

describe('generator module method', () =>{
  test('does not get mocked', () => {
    expect(mod.syncMethod).toBeDefined();
    expect(mod.asyncMethod).toBeDefined();
    expect(mod.generatorMethod).toBeDefined();
  });

  test('Mocked generator function is of type generator', () => {
    expect(Object.prototype.toString.apply(mod.syncMethod)).toBe('[object Function]');
    expect(Object.prototype.toString.apply(mod.generatorMethod)).toBe('[object GeneratorFunction]');
  });
});

Async Functions might also want to have a similar fix:

// The line below fails
expect(Object.prototype.toString.apply(mod.asyncMethod)).toBe('[object AsyncFunction]');

[mjesun]

That makes sense I think. In #6011 I'm introducing the concept of subtype into the mocker, to fix a React behavior (it's currently an RFC, as setting React specific code in Jest is a bit tricky, but we'll see).

Maybe you could rely on that to add async and generator to the list of subtypes, and use that when building the mock. WDYT?

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