enums.ts

interface Action {
    type: string;
}

enum UserActions {
    ADD_USER = 'ADD_USER',
    DELETE_USER = 'DELETE_USER'
}


interface User {
    id: number;
    name: string;
}

interface AddUserAction extends Action {
    type: UserActions.ADD_USER,
    payload: User
}

interface DeleteUserAction extends Action {
    type: UserActions.DELETE_USER,
    payload: number;
}


type UserAction =
    AddUserAction |
    DeleteUserAction


function userReducer(state: User[] = [], action: UserAction): User[] {
    switch (action.type) {
        case UserActions.ADD_USER:
          return state.concat([action.payload]);
        case UserActions.DELETE_USER:
            return state.filter(user => user.id === action.payload);
        default:
            return state;

    }

}


userReducer([], {type: UserActions.ADD_USER, payload: {id: 1, name: 'foo'}})