Listening for changes is the core purpose of observables, so Legend-State provides many options.


onChange listens to an observable for any changes anywhere within it. Use this as specifically as possible because it will fire notifications for every change recursively up the tree.

const obs = observable({ text: 'hi' })

obs.text.onChange(({ value }) => console.log('text changed to', value))
obs.onChange(({ value }) => console.log('obs changed to', value))


// Log: text changed to "hello"
// Log: obs changed to { text: "hello" }

onChange has some extra options for more advanced use:

  1. getPrevious: Function to compare with the previous value. It is a function to prevent the performance cost of cloning objects unnecessarily.
  2. changes: Array of all of the changes to this observable in the latest batch. This is mainly for the persistence plugins to know what to sync/update, but it may be good for other uses too.
  3. trackingType: Whether to track only shallow changes
  4. initial: Whether to run the callback immediately with the current value
// Full example
obs.onChange(({ value, getPrevious, changes }) => {
    const prev = getPrevious();
    changes.forEach(({ path, valueAtPath, prevAtPath }) => {
        console.log(valueAtPath, 'changed at', path, 'from', prevAtPath)
}, { initial: true, trackingType: true })

Dispose of listeners

Listening to an observable returns a dispose function to stop listening. Just call it when you want to stop listening.

const obs = observable({ text: 'hello' })

const onChange = () => { ... }

const dispose = obs.text.onChange(onChange)

// Cancel listening manually


observe can run arbitrary code when observables change, and automatically tracks the observables accessed while running, so it will update whenever any accessed observable changes.

This can be useful to use multiple observables at once, for the benefit of cleanup effects, or if you just like it more than onChange 😎.

The callback parameter has some useful properties:

  • num: How many times it's run. Use this to do something only the first time or not the first time.
  • previous: The previous value, which will be undefined on the first run and set to the return value
  • cancel: Set to true to not track the observables
  • onCleanup: A function to call before running the selector again

observe has an optional second reaction parameter which will run after the selector, and does not track changes. This can be useful for observing an event or a single observable.

import { observe, observable } from "@legendapp/state"
const state = observable({ isOnline: false, toasts: [] })

const dispose = observe((e) => {
    // This observe will automatically track state.isOnline for changes
    if (!state.isOnline.get()) {
        // Show an "Offline" toast when offline
        const toast = { id: 'offline', text: 'Offline', color: 'red' }

        // Remove the toast when the observe is re-run, which will be when isOnline becomes true
        e.onCleanup = () => state.toasts.splice(state.toasts.indexOf(toast), 1)

// Cancel the observe

Or use the second parameter to run a reaction when a selector changes. It has an additional value parameter, which contains the value of the selector.

// Observe the return value of a selector and observe all accessed observables
observe(state.isOnline, (e) => {
    console.log('Online status', e.value)
// Observe the return value of a selector and observe all accessed observables
observe(() => state.isOnline.get() && state.user.get(), (e) => {
    console.log('Signed in status', e.value)


when runs the given function only once when the predicate returns a truthy value, and automatically tracks the observables accessed while running the predicate so it will update whenever one of them changes. When the value becomes truthy it will call the function and dispose the listeners. If not given a function it will return a promise that resolves when the predicate returns a truthy value.

The predicate can either be an observable or a function.

import { when } from "@legendapp/state"

const obs = observable({ ok: false })

// Option 1: Promise
await when(obs.ok)

// Option 2: callback
const dispose = when(() => obs.ok.get(), () => console.log("Don't worry, it's ok"))

// Cancel listening manually


computed automatically tracks the observables accessed while computing, so you can return a computed value based on multiple observables, and it will update whenever one of them changes.

computed is lazy so it won't run the compute function until you get() the value the first time.

import { computed } from "@legendapp/state"

const obs = observable({ test: 10, test2: 20 })
const computed = computed(() => obs.test.get() + obs.test2.get())
// computed.get() === 30

// computed.get() === 25


event works like an observable without a value. You can listen for changes as usual, and dispatch it manually whenever you want. This can be useful for simple events with no value, like onClosed.

import { event } from "@legendapp/state"

const onClosed = event()

// Simply pass a callback to the `on` function
onClosed.on(() => { ... })

// Or use it with 'onChange' like other observables
onClosed.onChange(() => { ... }))

// Dispatch the event to call listeners