Todo App


This tutorial assumes you have gone through the Github Issues Viewer tutorial, and that you are already familiar with both RxJS and React.

The purpose of the tutorial is to introduce the most important APIs of the @react-rxjs/utils package, and for that we are going to build a simple todo-list application. Our app will be able to do the following:

  • Add todo items
  • Edit todo items
  • Delete todo items
  • Filter todo items
  • Display useful stats

Capturing user input

The first thing that we should do is to capture the events triggered by the user. Let's create a few Subjects for this. Also, it's probably best if our presentational components don't know about the existence of these Subjects. So we will also create a set of functions that capture the user-evens and push then into the Subjects:

import { Subject } from "rxjs"
const newTodo$ = new Subject<string>()
export const onNewTodo = (text: string) => text && newTodo$.next(text)
const editTodo$ = new Subject<{ id: number; text: string }>()
export const onEditTodo = (id: number, text: string) =>
editTodo$.next({ id, text })
const toggleTodo$ = new Subject<number>()
export const onToggleTodo = (id: number) => toggleTodo$.next(id)
const deleteTodo$ = new Subject<number>()
export const onDeleteTodo = (id: number) => deleteTodo$.next(id)

Creating a single stream for all the user events

It would be very convenient to have a merged stream with all those events. However, if we did a traditional merge, then it would be very challenging to know the origin of each event.

That's why @react-rxjs/utils exposes the mergeWithKey operator. Let's use it:

const todoActions$ = mergeWithKey({
add: newTodo$.pipe(map((text, id) => ({ id, text }))),
edit: editTodo$,
toggle: toggleTodo$.pipe(map(id => ({ id }))),
delete: deleteTodo$.pipe(map(id => ({ id })))

Which is basically the same as doing this (but a lot shorter, of course ๐Ÿ˜„):

const todoActions$ = merge(
newTodo$.pipe(map(text, id) => ({
type: "add" as const
payload: { id, text },
editTodo$.pipe(map(payload => ({
type: "edit" as const,
toggleTodo$.pipe(map(id => ({
type: "toggle" as const,
payload: { id },
deleteTodo$.pipe(map(id => ({
type: "delete" as const,
payload: { id },

Creating a stream for each todo

Now that we have put all the streams together, let's create a stream for each todo. And for that, we will be using another operator from @react-rxjs/utils: the split operator:

type Todo = { id: number, text: string, done: boolean }
const todos$: Observable<GroupedObservable<number, Todo>> = todoActions$.pipe(
event =>,
(event$, id) => event$.pipe(
takeWhile(event => event.type !== 'delete'),
(state, action) => {
switch (action.type) {
case "add":
case "edit":
return { ...state, text: action.payload.text }
case "toggle":
return { ...state, done: !state.done}
return state
{ id, text: "", done: false } as Todo

As you can see split is very similar to the groupBy operator that's exposed from RxJS. However, there are some important differences:

  • The first difference is that split doesn't have a "duration selector" argument for determining the duration of an inner stream. Once an inner stream completes, split will forget about it, meaning that it will remove it from its internal cache. Therefore, if afterwords the source emits a value with the same key, then split will create (and emit) a new GroupedObservable.

  • Another important difference is the second argument of split, which allows you to create a complex inner stream that will become the "grouped" stream that is emitted.

  • Also, this returned stream is enhanced with a shareReplay(1), and split internally subscribes to it as soon as it is created to ensure that the consumer always has the latest value.

Collecting the GroupedObservables

Our todos$ variable is an Observable of GroupedObservables<number, Todo> and that in itself is not very useful. It would be a lot more convenient to have an Observable of Map<number, Todo>. Which is exactly what the collectValues operator is for. Let's try it:

const todosMap$: Observable<Map<number, Todo>> = todos$.pipe(collectValues())

And with this we are ready to start wiring things up.

Wiring up a basic version

Let's start with the top-level component:

const [useTodos] = bind(todosMap$.pipe(map(todosMap => [...todosMap.values()])))
function TodoList() {
const todoList = useTodos()
return (
{/* <TodoListStats /> */}
{/* <TodoListFilters /> */}
<TodoItemCreator />
{ => (
<TodoItem key={} item={todoItem} />

Next, let's implement the TodoItemCreator:

function TodoItemCreator() {
const [inputValue, setInputValue] = useState('');
const addItem = () => {
const onChange = ({target: {value}}) => {
return (
<input type="text" value={inputValue} onChange={onChange} />
<button onClick={addItem}>Add</button>

And finally, the TodoItem component:

function TodoItem({item}) {
const editItemText = ({target: {value}}) => {
onEditTodo(, value)
const toggleItemCompletion = () => {
const deleteItem = () => {
return (
<input type="text" value={item.text} onChange={editItemText} />
<button onClick={deleteItem}>X</button>

That's it! We've already got the basic version working. Now let's add the filters and the stats.

Adding filters

As we already know, we will need to capture the filter selected by the user:

export enum FilterType {
All = "all",
Done = "done",
Pending = "pending"
const selectedFilter$ = new Subject<FilterType>()
export const onSelectFilter = (type: FilterType) => {

Next, let's create a hook and a stream for the current filter:

const [useCurrentFilter, currentFilter$] = bind(

Also, let's make sure that our useTodos hook takes this into account:

const todosList$ = todosMap$.pipe(
map(todosMap => [...todosMap.values()]),
shareLatest(), // We are using shareLatest because the stats will also consume it
const [useTodos] = bind(
combineLatest(todosList$, currentFilter$).pipe(
map(([todos, currentFilter]) => {
if (currentFilter === FilterType.All) {
return todos
const isDone = currentFilter === FilterType.Done
return todos.filter(todo => todo.done === isDone)

Time to implement the TodoListFilters component:

function TodoListFilters() {
const filter = useCurrentFilter()
const updateFilter = ({target: {value}}) => {
return (
<select value={filter} onChange={updateFilter}>
<option value={FilterType.All}>All</option>
<option value={FilterType.Done}>Completed</option>
<option value={FilterType.Pending}>Uncompleted</option>

Adding stats

We will be showing the following stats:

  • Total number of todo items
  • Total number of completed items
  • Total number of uncompleted items
  • Percentage of items completed

Let's create a useTodosStats for it:

const [useTodosStats] = bind(
todosList$.pipe(map(todosList => {
const nTotal = todosList.length
const nCompleted = todosList.filter((item) => item.done).length
const nUncompleted = nTotal - nCompleted
const percentCompleted =
nTotal === 0 ? 0 : Math.round((nCompleted / nTotal) * 100)
return {

And now let's use this hook in the TodoListStats component:

function TodoListStats() {
const { nTotal, nCompleted, nUncompleted, percentCompleted } = useTodosStats()
return (
<li>Total items: {nTotal}</li>
<li>Items completed: {nCompleted}</li>
<li>Items not completed: {nUncompleted}</li>
<li>Percent completed: {percentCompleted}</li>


The result of this tutorial can be seen in this CodeSandbox: