jest-each: Support accessing attributes in name interpolation

[jackwilsdon]

🚀 Feature Proposal

jest-each should support accessing attributes on objects when generating a description for a test.

For example;

each`
 object           | expectedValue
 ${{ value: 5 }}  | ${5}
 ${{ value: 10 }} | ${10}
`.test('object with a value of ${object.value} has a value of $expectedValue', () => ...)

Admittedly this is not a great example, but I'm sure you can get the idea.

Motivation

I came across this whilst trying to test my Redux reducers using jest-each;

// Actions
const increment = () => ({ type: 'increment' });
const decrement = () => ({ type: 'decrement' });

// Reducer
const reducer = (state = 0, { type }) => {
  switch (type) {
    case 'increment':
      return state + 1;
    case 'decrement':
      return state - 1;
    default:
      return state;
  }
};

each`
  state | action         | nextState
  ${0}  | ${increment()} | ${1}
  ${0}  | ${decrement()} | ${-1}
`.test(
  'returns $nextState when the state is $state and ${action.type} is dispatched',
  ({ state, action, nextState }) => {
    console.log(state, action, nextState);
    expect(3).toEqual(4);
  },
);

This would be a useful feature, as it would allow generation of descriptions for tests using object variables.

Example

See motiviation.

Pitch

Why does this feature belong in the Jest core platform?

It is an extension of an existing piece of functionality which can't be easily added (unless I'm missing something) without forking the library.

[mattphillips]

Hey @jackwilsdon I really like this idea.

I've gone ahead and implemented this over in #6457 with a slightly different syntax.

Instead of:
${action.type}

I've gone with:
$action.type

Feel free to give feedback on the PR 😄

[mattphillips]

BTW @jackwilsdon if you are using Jest@23 you can now use jest-each natively with: test.each and describe.each see docs here

[jackwilsdon]

@mattphillips great! Much appreciated 😁. The reason I suggested the ${} syntax was to avoid ambiguity, for example; This is a $sentence.I hope it comes out alright! (we'd end up getting $sentence.I instead of just $sentence in this example).

I understand that's not the best example in the world, but I'm sure you get the idea.

Cheers! 🎉

[szafirov]

Is there a special syntax to print the value of something like a.b[0]?
It currently prints the value of a.b followed by '[0]' instead.

[jackwilsdon]

Have you tried a.b.0?

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