As in "expect generator...". An assertion / snapshot library for testing iterators and generators. It was designed for, and works particularly well with, redux-saga but can be used for anything that uses generators.

function* myEffect(fakeAction) {
  const fooIds = yield select(fooIds);
  yield put(foosLoading());
  const results = yield call(apiFetch, options);
}

import expectGen from 'expect-gen';

it('runs effect with fakeFooIds and fakeResults', () => {
  expectGen(myEffect, fakeAction)
    // asserts step yields first `select(fooIds)`
    .yields(
      select(fooIds),
      // `fakeFooIds` gets pass into the following `next`
      fakeFooIds
    )
    .yields(
      put(foosLoading())
    )
    .yields(
      call(apiFetch, options),
      fakeResults
    )
    .finishes()
    .run();
});

it('runs effect with fakeFooIds and fakeResults', () => {
  const snapshot = expectGen(myEffect, fakeAction)
    // Doesn't run assertion for `next`
    .next(fakeFooIds)
    .next()
    .yields(
      call(apiFetch, options),
      fakeResults
    )
    .finishes()
    .toJSON();

  expect(snapshot).toMatchSnapshot();
});

expectGen(generator, [...args])

• Calls the first argument with the rest of the arguments
• Returns instance of StepManager

expectGen returns an instance StepManager. It gives us a set of chainable methods:

expectGen(generator, [...args])
  .yields(expectedValue, [result])

• Runs an assertion that iterator will yield the expectedValue
• Passes result into generator's next method
• Returns same instance StepManager

expectGen(generator, [...args])
  .next([result])

• Passes result into generator's next method
• Makes no assertion
• Returns same instance StepManager

expectGen(generator, [...args])
  .throws(error)

• Throws error into generator
• Checks that iterator throws an uncaught exception
• Returns same instance StepManager

expectGen(generator, [...args])
  .catches(error, [expectedValue])

• Throws error into generator
• Runs an assertion that thrown iterator will yield the expectedValue
• Returns same instance StepManager

expectGen(generator, [...args])
  .catchesAndFinishes(error, [result])

• Throws error into generator
• Asserts that generator finishes with result
• Returns same instance StepManager

expectGen(generator, [...args])
  ...
  .finishes([result])

• Asserts that generator finishes with result
• Returns same instance StepManager

expectGen(generator, [...args])
  ...
  .run()

• Executes all assertions
• Returns results of all generator steps as an array

expectGen(generator, [...args])
  ...
  .toJSON()

• Convert results from run() into a JSON object

Expect Gen is heading towards v1. The following items still need to go in before then. Please open issues for other v1 requirements you find.

• Allow custom assertion libraries
• Allow annotations to be added to assertions
• Add assert method

Expect Gen is a second generation version of generator-test-runner. Many of the concepts used here originated there.