Use default reducer state value as initial state when using useReducer hook

[kangaroooh]

Use default reducer state value as initial state when using useReducer hook

Based on discussion from issue facebook#14542.

If we provide the following reducer:

function reducer(state = 5, { type }) {
  switch (type) {
    case 'INCREMENT':
      return state + 1;
    case 'DECREMENT':
      return state - 1;
    default:
      return state;
  }
}

Then in the component:

function Counter() {
  const [count] = useReducer(reducer);
  return <span>{count}</span>;
}

We don't get count equal 5 in the initial render.

It works if we provide the initialState as second argument for useReducer:

const [count] = useReducer(reducer, 5);

or like @ckknight mentioned if we call an initialization action:

useReducer(reducer, undefined, { type: 'INIT' });

However I think it's a little redundant because we already set the default state value in the reducer function.

This pull request add some changes to use the default state value from the reducer function if no initialState was provided to useReducer.

I don't know if this behaviour was discussed during the implementation of useReducer, but if this is not the desired behaviour for useReducer feel free to close this pull request.

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