How to set a property as an array i initial state?

问题

In my reducer I set initial state by:

const initialState = fromJS({
  results: [],
});

However if I try to print results by

initialState.get('results')

I get an immutable Map.

On the other hand if in my reducer (listening to an action) I set the array via

...
case LOAD_SUCCESS:
  return state
     .set('results', []);
...

the array will be an actual (non-Immutable) array after executing:

state.get('results')

(e.g. in a selector defined via reselect)

Why?

回答1:

From the fromJS docs:

Deeply converts plain JS objects and arrays to Immutable Maps and Lists.

That said, fromJS({ results: [] }) is equal to Map({ results: List([])})

as soon as you call state.set('results', []), you replace List([]) with plain array [].

This is a classical trap for new starters.

The way I see it is you have to choose to either always use List there, or plain array.

List way:

Initialize: const state = fromJS({results: []})

Reduce: return state.update("results", list => list.clear())

Array way #1:

Initialize: const state = fromJS({results: null})

INIT reduce (dispatched once, to init state) return state.set("results", [])

Reduce: return state.set("results", [1, 2, 3])

Array way #2:

Initialize: const state = Map({results: []})

Reduce: return state.set("results", [1, 2, 3])

---

I'd recommend to always use Array way #2. It adds more code, as you have to control List / Map for each state's field, but it guarantees you that you get exactly what you want.

In the end it comes something like this:

const initialState = Map({ results: [], some_field_as_list: List([]), some_field_as_map: Map([]), ... // etc });

回答2:

fromJS({ results: [] }) is equal to Map({ results: List([])}),

Using state.set('results', []) you replace List([]) with plain []

I found this helpful in this case:

• Initialize: const state = fromJS({results: []})

• Reduce: return state.set('results', List([1,2,3]));

It was better than using plain array because later you can use setIn function editing the same state variable:

• return state.setIn(['results',state.get("results").size], nextElement);

or

• return state.setIn(['results',-1], lastElement);

来源：https://stackoverflow.com/questions/43716784/how-to-set-a-property-as-an-array-i-initial-state