Redux actions using TypeScript discriminated unions

redux-actions-with-typescript-discriminated-unions.ts

// Redux actions using TypeScript discriminated unions
//
// Source: https://github.com/rjz/typescript-react-redux/

// Actions
export type Action =
   { type: 'INCREMENT_COUNTER', delta: number }
|  { type: 'RESET_COUNTER' }


type Counter = { value : number }

// Reducer
function counter (state: Counter = { value: 0 }, action: Action): Counter {
  switch (action.type) {
    
    case 'INCREMENT_COUNTER':
      // Since `INCREMENT_COUNTER` only matches one type in the union, the
      // compiler allows us to extract additional fields (i.e., `delta`) from
      // the action without casting.
      const { delta } = action
      return { value: state.value + delta }

    case 'RESET_COUNTER':
      return { value: 0 }

    // compilation will fail for any types / `case`s that aren't declared in
    // the `Action` union.

    default:
      return state
  }
}