Discussion on: Why does typescript not conditionally pick the right type in a union?

[Thibault Maekelbergh]

OK, I realized it shortly after posting. However, how would you ever handle a typical case like a state reducer (in Redux for example) where your payloads can contain different subsets of data but they occur via events?

interface PayloadA {
  propA: string;
}

interface PayloadB {
  propB: number;
}

interface Action = {
  type: string;
  payload: PayloadA | PayloadB;
}

function reducer(state = {}, action: Action) {
  switch(action.type) {
    case 'ADD':
       return { …state, action.payload.propA }
    case 'UPDATE':
      return { …state, action.payload.propB }
    default: return state;
  }
}

TS always warns that propB would not be available in type ADD being dispatched and propA not being available in UPDATE being dispatched.

[Pragmatic Maciej]

So the proper typying of this case is:

interface PayloadA {
  type: 'ADD';
  propA: string;
}

interface PayloadB {
  type: 'UPDATE';
  propB: number;
}

type Action = PayloadA | PayloadB 

We need to join type with prop, in other way TS is not able to understand what property is in add action or update action. Such construct is known as sum type or discriminated union.

[Thibault Maekelbergh]

Thanks, I'll try this