Getting Started

Install Legend-State

npm i @legendapp/state

Core concepts


You can put anything in an observable: primitives, deeply nested objects, arrays, functions, etc... Observables work just like normal objects so you can interact with them without any extra complication. Just call get() to get a value and set(...) to modify it.

import { observable } from "@legendapp/state"

const state$ = observable({ text: 'hello', obj: { value: 10 } })

const text = state$.text.get() // 'hello'
state$.obj.value.get() === 10 // true

// Use the set function anywhere

// Easily modify the previous value
state$.text.set(text => text + ' there')

Read more

Observing observables

You can subscribe to changes anywhere in the hierarchy of an object with onChange(...), and then any change to that node will call the listener.

const state$ = observable({ settings: { theme: 'light' }, array: [{text: 'hi'}] })

// Listen to observable directly
state$.settings.theme.onChange(({ value }) => console.log('Theme is', value))

The core power of Legend-State is the "observing contexts" in which any call to get() will subscribe the observer to that node, and the observer will re-run whenever it changes. This includes observe, when, computed, useSelector, observer, <Memo>, <Computed>, and reactive props (we'll get to all of that later).

// This will re-run whenever accessed observables change
observe(() => {
    console.log('Theme is', state$.settings.theme.get())

// when waits for a value to become truthy.
await when(() => state$.settings.theme.get() === 'dark')

// an observable can be computed based on other observables
const isDark$ = observable(() => state$.settings.theme.get() === 'dark')

Read more


Many of the functions in Legend-State take a Selector, which can be either an observable or a function that returns a value based on observables. The selector is run in an observing context so that get() tracks an observable for changes. Whenever an observable changes, it re-runs the function.

Using when as an example of using Selectors:

const isSignedIn$ = observable(false)
const isOnline$ = observable(false)

// A selector can be just an observable, which will be tracked for changes
await when(isSignedIn$)

// Or selector can be a function which tracks all get() callschanges
await when(() => isSignedIn$.get() && isOnline$.get())

Getting started

1. Configure your options

Legend-State is designed to have a lean core that allows you and your team to add additional features, so it has configuration functions to add features as you like. If you are using React we recommend at least adding the use() function - it is the recommended way to use observables in React as you will see in the examples. You only need to do this once in your app's entry point.

import { enableReactUse } from '@legendapp/state/config/enableReactUse';
enableReactUse() // This adds the use() function to observables

There are also more options such as enabling getting/setting values directly with a $ property. See configuring for more details.

2. Create global state

Observables are designed to contain large hierarchy, and many teams like to have one large global store.

import { observable } from '@legendapp/state';

export const state$ = observable({
    UI: {
        windowSize: undefined as { width: number, height: number },
        activeTab: 'home' as 'home' | 'user' | 'profile',
    settings: {
        theme: 'light' as 'light' | 'dark',
        fontSize: 14,
    todos: []

Or if you prefer to have multiple individual atoms in multiple files, you can do that too.

// settings.ts
export const theme = observable('light')
export const fontSize = observable(14)

// UIState.ts
export const uiState = observable({
    windowSize: undefined as { width: number, height: number },
    activeTab: 'home' as 'home' | 'user' | 'profile',

3. Use in React

Head over to React Introduction for a detailed guide to getting started in React.

4. Persistence plugins

Use persistObservable to automatically persist state using any kind of local or remote storage. Legend-State includes local providers for Local Storage on web and react-native-mmkv in React Native, with more local and remote providers coming soon. Use configureObservablePersistence to set default providers for all persisted observables, or you can set them individually if they need to be different.

The given observables will be populated with their persisted state immediately after calling persistObservable.

// Global configuration
    // Use Local Storage on web
    persistLocal: ObservablePersistLocalStorage
    // Use react-native-mmkv in React Native
    persistLocal: ObservablePersistMMKV

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

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

Read more