This is a type-safe version of the To-Do list example in Redux's documentation. Everything is typed and should pass in 'strict' mode'. It uses some advanced typescript features such as "keyof".

redux-typescript-todo-list-example.ts

import { createStore, combineReducers, Reducer } from "redux";

///////////////////////////////////////////////////////////////////////
// action types

// first, define all the action types
const ADD_TODO = "ADD_TODO";
const COMPLETE_TODO = "COMPLETE_TODO";
const REMOVE_TODO = "REMOVE_TODO";
const SET_VISIBILITY_FILTER = "SET_VISIBILITY_FILTER";

const VisibilityFilters = {
  SHOW_ALL: "SHOW_ALL",
  SHOW_COMPLETED: "SHOW_COMPLETED",
  SHOW_ACTIVE: "SHOW_ACTIVE"
} as const;

// we want to create a type that allows "SHOW_ALL", "SHOW_COMPLETED", and "SHOW_ACTIVE".
// We could just do: type IVisibilityFilter "SHOW_ALL" | "SHOW_COMPLETED" | "SHOW_ACTIVE"
// but that is repetitive. 
// 
// Instead, we can take advantage of two TS keywords, `keyof` and `typeof`.
// https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-1.html
// typeof VisibilityFilters is something like:
//   interface {
//     SHOW_ALL: string
//     SHOW_COMPLETED: string
//     SHOW_ACTIVE: string
//   }
//
// so keyof typeof VisibilityFilters === "SHOW_ALL" | "SHOW_COMPLETED" | "SHOW_ACTIVE"
type IVisibilityFilter = keyof typeof VisibilityFilters; 

// now, define the interfaces each action payload. the only require element for each is 'type'. 

interface IAddTodoAction {
  type: typeof ADD_TODO; // can't do "string", if we want proper type detection with switch statements.
  id: number;
  text: string;
}

interface ICompleteTodoAction {
  type: typeof COMPLETE_TODO;
  id: number;
}

interface IRemoveTodoAction {
  type: typeof REMOVE_TODO;
  id: number;
}

interface ISetVisibilityFilterAction {
  type: typeof SET_VISIBILITY_FILTER;
  visibilityFilter: keyof typeof VisibilityFilters;
}

// we combine all into a single type.
type IAction = IAddTodoAction | ICompleteTodoAction | IRemoveTodoAction | ISetVisibilityFilterAction;

///////////////////////////////////////////////////////////////////////
// action producers

function addTodo(text: string, id: number): IAction {
  return {
    type: ADD_TODO,
    id: id,
    text
  };
}

function completeTodo(id: number): IAction {
  return {
    type: COMPLETE_TODO,
    id
  };
}

function removeTodo(id: number): IAction {
  return {
    type: REMOVE_TODO,
    id
  };
}

function setVisibilityFilter(visibilityFilter: IVisibilityFilter): IAction {
  return {
    type: SET_VISIBILITY_FILTER,
    visibilityFilter
  };
}

///////////////////////////////////////////////////////////////////////
// state defs

type ITodos = Array<{ text: string; id: number; complete: boolean }>;

interface IState {
  todos: ITodos;
  visibilityFilter: IVisibilityFilter;
}

///////////////////////////////////////////////////////////////////////
// reducers

const todos: Reducer<ITodos, IAction> = (todos: ITodos = [], action: IAction) => {
  switch (action.type) {
    case ADD_TODO:
      return [...todos, { text: action.text, id: action.id, complete: false }];
    case COMPLETE_TODO:
      return todos.map(todo => {
        if (todo.id === action.id) {
          return Object.assign({}, todo, {
            complete: true
          });
        }
        return todo;
      });
    case REMOVE_TODO:
      return todos.filter(todo => todo.id !== action.id);
    default:
      return todos;
  }
};

const visibilityFilter: Reducer<IVisibilityFilter, IAction> = (
  visibilityFilter: IVisibilityFilter = "SHOW_ALL",
  action: IAction
) => {
  switch (action.type) {
    case SET_VISIBILITY_FILTER:
      return action.visibilityFilter;
    default:
      return visibilityFilter;
  }
};

// combine them
const todoApp = combineReducers<IState>({
  todos: todos,
  visibilityFilter: visibilityFilter
});

///////////////////////////////////////////////////////////////////////
// use it

const store = createStore(todoApp, {
  // initial value. leave createStore's second argument out if you want to use the values from the default values in the reducers
  todos: [],
  visibilityFilter: "SHOW_COMPLETED"
});
const unsubscribe = store.subscribe(() => console.log(store.getState()));

store.dispatch(addTodo("Test 1", 1));

// { todos: [ { text: 'Test 1', id: 1, complete: false } ],
//   visibilityFilter: 'SHOW_COMPLETED' }

store.dispatch(addTodo("Test 2", 2));
// { todos:
//    [ { text: 'Test 1', id: 1, complete: false },
//      { text: 'Test 2', id: 2, complete: false } ],
//   visibilityFilter: 'SHOW_COMPLETED' }

store.dispatch(addTodo("Test 3", 3));
// { todos:
//    [ { text: 'Test 1', id: 1, complete: false },
//      { text: 'Test 2', id: 2, complete: false },
//      { text: 'Test 3', id: 3, complete: false } ],
//   visibilityFilter: 'SHOW_COMPLETED' }

store.dispatch(completeTodo(3));
// { todos:
//    [ { text: 'Test 1', id: 1, complete: false },
//      { text: 'Test 2', id: 2, complete: false },
//      { text: 'Test 3', id: 3, complete: true } ],
//   visibilityFilter: 'SHOW_COMPLETED' }

store.dispatch(removeTodo(2));
// { todos:
//    [ { text: 'Test 1', id: 1, complete: false },
//      { text: 'Test 3', id: 3, complete: true } ],
//   visibilityFilter: 'SHOW_COMPLETED' }

store.dispatch(setVisibilityFilter('SHOW_ACTIVE'));
// { todos:
//    [ { text: 'Test 1', id: 1, complete: false },
//      { text: 'Test 3', id: 3, complete: true } ],
//   visibilityFilter: 'SHOW_ACTIVE' }