Introduction

Legend-State is a super fast and powerful state library for modern JavaScript apps with four primary goals:

1. 🦄 As easy as possible to use

There is no boilerplate and there are no contexts, actions, reducers, dispatchers, sagas, thunks, or epics. It doesn't modify your data at all, and you can just call get() to get the raw data and set() to change it.

In React components you can call use() on any observable to get the raw data and automatically re-render whenever it changes.

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

// Just get and set
const theme = state$.settings.theme.get();
state$.settings.theme.set('light')

// observe re-runs when accessed observables change
observe(() => {
    console.log(state.settings.theme.get())
})

const Component = function Component() {
    // use() makes this component re-render whenever it changes
    const theme = state$.settings.theme.use()

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

2. ⚡️ The fastest React state library

Legend-State beats every other state library on just about every metric and is so optimized for arrays that it even beats vanilla JS on the "swap" and "replace all rows" benchmarks. At only 4kb and with the massive reduction in boilerplate code, you'll have big savings in file size too.

See Fast 🔥 for more details of why Legend-State is so fast.

3. 🔥 Fine-grained reactivity for minimal renders

Legend-State lets you make your renders super fine-grained allowing only the leaves of your component tree to re-render when required, thus making your apps go faster 🔥, and removing unnecessary overhead from React's render cycle.

function Normal() {
    const [count, setCount] = useState(0);

    useInterval(() => {
        setCount(v => v + 1)
    }, 600)

    // This re-renders when count changes
    return (
        <div>
            Count: {count}
        </div>
    )
}
function FineGrained() {
    const count$ = useObservable(0)

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

    // The text updates itself so the component doesn't re-render
    return (
        <div>
            Count: <Memo>{count$}</Memo>
        </div>
    )
}

Normal

Renders: 1

Count: 1

Fine-grained

Renders: 1

Count:

4. 💾 Powerful persistence

Legend-State includes a powerful persistence plugin system for local caching and remote sync. It easily enables offline-first apps by tracking changes made while offline that save when coming online, managing conflict resolution, and syncing only small diffs. We use Legend-State as the sync systems in Legend and Bravely, so it is by necessity very full featured while being simple to set up.

Local persistence plugins for the browser and React Native are included, and a remote sync plugin for Firebase (coming soon).

import { ObservablePersistLocalStorage } from '@legendapp/state/persist-plugins/local-storage'
import { persistObservable } from '@legendapp/state/persist'

const state$ = observable({ store: { bigObject: { ... } } })

// Persist this observable
persistObservable(state$, {
    persistLocal: ObservablePersistLocalStorage,
    local: 'store' // Unique name
})

Install

npm

yarn

npm i @legendapp/state

Highlights

✨ Super easy to use 😌
✨ Super fast ⚡️
✨ Super small at 4kb 🐥
✨ Fine-grained reactivity 🔥
✨ No boilerplate
✨ Designed for maximum performance and scalability
✨ React components re-render only on changes
✨ Very strongly typed with TypeScript
✨ Persistence plugins for automatically saving/loading from storage
✨ State can be global or within components

The core is platform agnostic so you can use it in vanilla JS or any framework to create and listen to observables. It includes support for React and React Native, and has plugins for automatically persisting to storage.

Read more about why you'll love Legend-State ❤️

Example

This example shows an overview of what using Legend-State looks like. See Getting Started to dive into how it works.

import { observable, observe } from "@legendapp/state"
import { persistObservable } from "@legendapp/state/persist"

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

// get() returns the raw data
state$.settings.theme.get() === 'dark'

// observe re-runs when any observables change
observe(() => {
    console.log(state$.settings.theme.get())
})

// Assign to state$ with set
state$.settings.theme.set('light')

// Automatically persist state$. Refresh this page to try it.
persistObservable(state$, { local: 'exampleState' })

// Components re-render only when accessed observables change
// This is the code for the example on your right ----->
function Component() {
    const theme = state$.settings.theme.use()
    // state$.settings.theme is automatically tracked for changes

    const toggle = () => {
        state$.settings.theme.set(theme =>
            theme === 'dark' ? 'light' : 'dark'
        )
    }

    return (
        <div
            className={theme === 'dark' ? 'theme-dark' : 'theme-light'}
        >
            <div>Theme: {theme}</div>
            <Button onClick={toggle}>
                Toggle theme
            </Button>
        </div>
    )
}

Theme: light