Skip to main content

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 some Signals for this:

const [newTodo$, onNewTodo] = createSignal<string>();
const [editTodo$, onEditTodo] = createSignal<{ id: number; text: string }>();
const [toggleTodo$, onToggleTodo] = createSignal<number>();
const [deleteTodo$, onDeleteTodo] = createSignal<number>();

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 😄):

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 partitionByKey operator:

type Todo = { id: number; text: string; done: boolean };
const [todosMap, keyChanges$] = partitionByKey(
event =>,
(event$, id) =>
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

Now we have a function, todosMap, that returns an Observable of events associated with a given todo. partitionByKey transforms the source observable in a way similar to the groupBy operator that's exposed from RxJS. However, there are some important differences:

  • partitionByKey gives you a function that returns an Observable, rather than an Observable that emits Observables. It also provides an Observable that emits the keys that have changed (keyChanges$ in the code above).
  • partitionByKey has an optional third parameter which allows you to create a complex inner stream that will become the "grouped" stream that is returned.
  • This returned stream is enhanced with a shareLatest and partitionByKey internally subscribes to it as soon as it is created to ensure that the consumer always has the latest value.

Collecting the GroupedObservables

We now have a way of getting streams for each todo, and we have a stream (keyChanges$) that represents the keys that were added or removed. We should also like a stream that emits whenever the state of any todo changes, and gives us access to all of them. combineKeys() suits this purpose. Let's try it:

const todosMap$: Observable<Map<number, Todo>> = combineKeys(keyChanges$, todosMap);

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 have a basic version working.

Cutting React out of the state management game

What we've done so far is pretty neat, but there are a lot of unnecessary renders going on in our application. Editing any of the todos, for example, causes the whole list to re-render. To those with experience in React development, this hardly seems noteworthy—our state is our list of todos, it lives in our TodoList component, so of course it re-renders when that state changes. With React-RxJS, we can do better. Before we proceed with the remaining features, let's relieve React of its state management responsibilities altogether.

Take a look at the stream we've bound to the TodoList component:

const [useTodos] = bind(todosMap$.pipe(map(todosMap => [...todosMap.values()])));

This is just an Observable<Todo[]>, and it will emit every time any todo gets updated—triggering a TodoList render. In fact TodoList only needs to know which todos to display; rendering them according to their properties can be left up to the child component, TodoItem. Therefore let's bind a list of which todos exist. Luckily we already have a stream for that, returned above by partitionByKey. So:

const [useTodoIds] = bind(
map((set) => Array.from(set))

Simple! Now we edit our TodoList component to pass just the todo id:

 function TodoList() {
- const todoList = useTodos();
+ const todoIds = useTodos();

return (
<TodoListStats />
<TodoListFilters />
<TodoItemCreator />

- { => (
- <TodoItem key={} item={todoItem} />
- ))}
+ { => (
+ <TodoItem key={id} id={id} />
+ ))}

and teach TodoItem to get its state from the stream corresponding to that id, rather than from its parent component:

const TodoItem: React.FC<{ id: number }> = ({ id }) => {
const item = useTodo(id);

return( ... )

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$, onSelectFilter] = createSignal<FilterType>()

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

const [useCurrentFilter, currentFilter$] = bind(

Also, let's tell our TodoItems not to render if they've been filtered out:

 const TodoItem: React.FC<{ id: number }> = ({ id }) => {
const item = useTodo(id);
+ const currentFilter = useCurrentFilter();

return ( ... );

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: