Introducing Legend-State 1.0: Build faster apps faster

May 16, 2023

After almost a year of development and iterating, we're very excited to announce that Legend-State has reached version 1.0! What started as a simple upgrade of Legend's state/sync system turned into a collaboration with Bravely, then into a quest for the best possible performance, and finally into a fundamental rethinking of React's developer experience (DX).

So in its 1.0 version, Legend-State has four main benefits:

  1. ⚡️ The fastest React state library
  2. 🦄 Very easy to use
  3. 🔥 Natural fine-grained reactivity
  4. 💾 Built-in persistence

Building three large apps on Legend-State (Legend's React app and Bravely's React and React Native apps) gave us a lot of room to iterate towards an optimal DX, and to optimize performance in all sorts of scenarios. Now that they're released and running in production, we're confident that Legend-State is ready for you to build your apps.

👋 Introducing Legend-State

Legend-State is a super fast and powerful React state library based on Observables. You may have heard a lot about Signals recently - Observables are conceptually similar but more powerful: deep reactive objects that notify listeners whenever anything in them changes.

Because we can listen for changes anywhere within an object, a React component can update only when a specific value changes while a persistence plugin can update whenever anything in the whole tree changes. Legend-State's Observables use Proxy in a unique way, tracking by path in an object, which makes it extremely fast and doesn't modify the underlying data at all.

Legend-State beats other React state libraries in both speed and memory usage. An optimized mode for arrays can re-render only changed elements instead of the whole array, making it significantly faster than is usually possible with React - see it on the left side of this table from the krausest benchmark.

🦄 As easy as possible to use

There is no boilerplate and there are no contexts, actions, reducers, dispatchers, sagas, thunks, or epics. Just call get() to get the raw data and set() to change it.

In React there is an observer HOC or a useSelector hook to get your data. Just access observables and your components update themselves automatically.

// Create an observable object
const state = observable({ settings: { theme: 'dark' } })

// Just get and set
state.settings.theme.get() === 'dark'

// observe re-runs when accessed observables change
observe(() => {

// 1. Observer components automatically track observables and re-render when they change
const Component = observer(function Component() {
    const theme = state.settings.theme.get()

    return <div>Theme: {theme}</div>

// 2. Or the useSelector hook gets and tracks the observable, and returns the raw value
function Component2() {
    const count = useSelector(state.count);

    // Re-renders whenever count changes
    return <div>{count}</div>

While our primary goal is to achieve the fastest performance with the best possible DX, we're most excited about the easier way of working with React that it enables: to observe state changing rather than managing hooks re-running because of re-renders. You can now just skip that entire layer of React development entirely (if you want).

We're building apps much faster because the mental model is so much easier, and our apps have better performance without even thinking about it.

🔥 Fine-grained reactivity for minimal renders

We want our apps to be extremely fast and we also want them to be easy to build. So Legend-State makes fine-grained reactivity natural, resulting in fewer and smaller re-renders while actually removing complexity.

  • Text elements re-render themselves when they change
  • Reactive props update themselves when state changes them
  • Control-flow components re-render themselves when needed
  • Two-way binding for input elements

And they all do that without needing to re-render the component itself. Check out our previous post Making React fast by default and truly reactive for more on the motivation and benefits of this approach. But for now here's an example:

function Component({ children }) {
  // This component only renders once
  const state = useObservable({ show: false, toggles: 0, text: 'Change me' })

  useInterval(() => { => !v)
    state.numToggles.set(v => v + 1)
  }, 1000)

  // Props ending in $ bind the prop to the observable
  // for tiny targeted re-renders on changes
  return (
      <Reactive.input $value={state.text} />
        $className={() => (
          state.text.get().includes('Legend') && 'font-bold text-blue-500'
        Modal toggles: <Memo>{state.numToggles}</Memo>
        else={<div>Not showing modal</div>}
Renders: 1
Change me
Modal toggles: 0
Not showing modal

Of course you can just use observer or useSelector if you prefer, but this micro-updating is a great way to improve performance and feels very natural once you get used to it.

💾 Built-in Persistence

Application state almost always needs to be saved and synced, so Legend-State builds that in. You can persist an entire observable object or subtree with a single command.

// The state is automatically loaded at startup and saved on any change. Easy 😌.
persistObservable(state, {
    local: 'State',
    persistLocal: ObservablePersistIndexedDB,

Version 1 ships with plugins for Local Storage and IndexedDB for web and react-native-mmkv for React Native.

We also have a Firebase Realtime Database plugin which is the backbone of the sync systems for Legend and Bravely that we'll release soon, with hopefully more sync services to come. With that, your entire sync system could look like this:

persistObservable(userData, {
    persistLocal: ObservablePersistLocalStorage,
    persistRemote: ObservablePersistFirebaseDatabase,
    local: 'User',
    remote: {
        firebase: {
            syncPath: (uid) => `/users/${uid}/`

Remote sync is very fully featured - it works offline and resolves conflicts when coming online, can sync only the changes since the last update, supports encryption and field transforming, and more... But we'll get to that in a future update.

🤟 Getting started

Check out the documentation or the GitHub repo to get started with Legend-State. We would love to hear from you on GitHub, or talk to LegendApp or me directly on Twitter.

Or if you just want to try it out right now, here's a sandbox you can play with:

import { useObservable, Memo } from '@legendapp/state/react'
import { useRef } from 'react'
import { useInterval } from './useInterval'

export default function Counter() {
  const renderCount = ++useRef(0).current;
  const count = useObservable(1)

  useInterval(() => {
    count.set(v => v + 1)
  }, 600)

  return (
        <div>Renders: {renderCount}</div>
        <div>Count: <Memo>{count}</Memo></div>

👉 What's next

We'll add our Firebase Realtime Database persistence plugin to the repo soon. Then we would like to add more persistence plugins for other services.

The fine-grained reactivity features are still a new paradigm even for us, so there's a lot more to explore there!

We have about a dozen helper observables like currentTime which updates every minute and hooks like useObservableNextRouter which enables easy hash-based routing in Next.js. This is just a start! We would love to have a massive library of helper observables and hooks, to reduce the amount of boilerplate required to build your apps.

If you'd like to help us with any of that please let us know on GitHub.