`useReducer` does not honor reducer function's default state value

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

Bug

What is the current behavior?

Please see the CodeSandBox here: https://codesandbox.io/s/v8y72ky0o5

Consider the following reducer:

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

Suppose we are trying to use the reducer in a component like so:

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

When rendered, count is undefined.

What is the expected behavior?

One would expect count to be equal to 5, because that is the default value for state in the reducer.

IMO, it would make sense if initialState argument for useReducer is undefined, to call reducer(undefined, undefined) to initialize the internal state.

Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?

16.7.0-alpha.2

[gaearon]

I think useReducer(reducer, undefined, { type: 'INIT' }) is what we suggest so that strong typing can work.

Edit: final API has changed, see below

[ckknight]

useReducer(reducer, undefined, { type: 'INIT' }) will work as you expect. It will execute the reducer initially, once, with a { type: 'INIT' } action, thus falling through to the default case. Because initialState is provided as undefined, it will appropriately default to 5 as specified in the reducer.