Valtio

Proxy state made simple.

The Valtio API is minimal, flexible, unopinionated and a touch magical. Valtio's proxy turns the object you pass it into a self-aware proxy, allowing fine-grained subscription and creativity in making state updates. In React, Valtio shines at render optimization. It is compatible with Suspense and React 18. Valtio is also a viable option in vanilla javascript applications.

Installation

npm i valtio

The to-do app example

1. proxy

Let's learn Valtio by coding a simple to-do app in React and Typescript. We'll start by creating some state using proxy.

import { proxy, useSnapshot } from 'valtio'

type Status = 'pending' | 'completed'
type Filter = Status | 'all'
type Todo = {
  description: string
  status: Status
  id: number
}

export const store = proxy<{ filter: Filter; todos: Todo[] }>({
  filter: 'all',
  todos: [],
})

2. useSnapshot

To access the data in this store, we'll use useSnapshot. The Todos component will rerender when the "todos" or "filter" properties are updated. Any other data we add to the proxy will be ignored.

const Todos = () => {
  const snap = useSnapshot(store)
  return (
    <ul>
      {snap.todos
        .filter(({ status }) => status === snap.filter || snap.filter === 'all')
        .map(({ description, status, id }) => {
          return (
            <li key={id}>
              <span data-status={status} className="description">
                {description}
              </span>
              <button className="remove">x</button>
            </li>
          )
        })}
    </ul>
  )
}

3. actions

Finally, we need to create, update, and delete our todos. To do so, we simply mutate properties on the store we created, not the snap. Commonly, these mutations are wrapped up in functions called actions.

const addTodo = (description: string) => {
  store.todos.push({
    description,
    status: 'pending',
    id: Date.now(),
  })
}

const removeTodo = (index: number) => {
  store.todos.splice(index, 1)
}

const toggleDone = (index: number, currentStatus: Status) => {
  const nextStatus = currentStatus === 'pending' ? 'completed' : 'pending'
  store.todos[index].status = nextStatus
}

const setFilter = (filter: Filter) => {
  store.filter = filter
}

Finally, we wire up these actions to our inputs and buttons - check the demo below for the full code.

<button className="remove" onClick={() => removeTodo(index)}>
  x
</button>

Codesandbox demo

Mutating state outside of components

In our first to-do app, Valtio enabled mutations without worrying about performance or "breaking" React. useSnapshot turned these mutations into immutable snapshots and optimized renders. But we could have easily used React's own state handling. Let's add a bit of complexity to our to-do app to see what else Valtio offers.

We will add a "timeLeft" property to a todo. Now each todo will tick down to zero and become "overdue" if not completed in time.

type Todo = {
  description: string
  status: Status
  id: number
  timeLeft: number
}

Among other changes, we will add a Countdown component to display the ticking time for each todo. We are using an advanced technique of passing a nested proxy object to useSnapshot. Alternatively, make this a dumb component by passing the todo's "timeLeft" as a prop.

import { useSnapshot } from 'valtio'
import { formatTimeDelta, calcTimeDelta } from './utils'
import { store } from './App'

export const Countdown = ({ index }: { index: number }) => {
  const snap = useSnapshot(store.todos[index])
  const delta = calcTimeDelta(snap.timeLeft)
  const { days, hours, minutes, seconds } = formatTimeDelta(delta)
  return (
    <span className="countdown-time">
      {delta.total < 0 ? '-' : ''}
      {days}
      {days ? ':' : ''}
      {hours}:{minutes}:{seconds}
    </span>
  )
}

Mutate in module scope

Instead of managing multiple timers inside of a React component, let's move these updates outside of React altogether by defining a recursive countdown function in module scope which will mutate the todos.

const countdown = (index: number) => {
  const todo = store.todos[index]
  // user removed todo case
  if (!todo) return
  // todo done of overdue case
  if (todo.status !== 'pending') {
    return
  }
  // time over
  if (todo.timeLeft < 1000) {
    todo.timeLeft = 0
    todo.status = 'overdue'
    return
  }
  setTimeout(() => {
    todo.timeLeft -= 1000
    countdown(index)
  }, 1000)
}

We can start the recursive countdown from an enhanced addTodo action.

const addTodo = (e: React.SyntheticEvent, reset: VoidFunction) => {
  e.preventDefault()
  const target = e.target as typeof e.target & {
    deadline: { value: Date }
    description: { value: string }
  }
  const deadline = target.deadline.value
  const description = target.description.value
  const now = Date.now()
  store.todos.push({
    description,
    status: 'pending',
    id: now,
    timeLeft: new Date(deadline).getTime() - now,
  })
  // clear the form
  reset()
  countdown(store.todos.length - 1)
}

Please see the rest of the changes in the demo below.

Subscribe in module scope

Being able to mutate state outside of components is a huge benefit. We can also subscribe to state changes in module scope. We will leave it to you to try this out. You might, for instance, persist your todos to local storage as in this example.

Codesandbox demo