xstate

What is xstate?

The xstate npm package is a library for creating, interpreting, and executing finite state machines and statecharts, as well as managing invocations of those machines as actors. It provides a robust framework for modeling and analyzing application logic in a composable and declarative way, which can help manage complex state logic in UIs, robotics, and other systems.

What are xstate's main functionalities?

Finite State Machines

This feature allows you to create simple finite state machines. The code sample demonstrates a toggle machine with two states: 'active' and 'inactive'.

{"import { Machine } from 'xstate';\n\nconst toggleMachine = Machine({\n  id: 'toggle',\n  initial: 'inactive',\n  states: {\n    inactive: { on: { TOGGLE: 'active' } },\n    active: { on: { TOGGLE: 'inactive' } }\n  }\n});"}

Statecharts

This feature allows you to create statecharts, which are an extension of state machines that can have hierarchical and parallel states. The code sample shows a traffic light machine with three states: 'green', 'yellow', and 'red'.

{"import { Machine } from 'xstate';\n\nconst lightMachine = Machine({\n  id: 'light',\n  initial: 'green',\n  states: {\n    green: { on: { TIMER: 'yellow' } },\n    yellow: { on: { TIMER: 'red' } },\n    red: { on: { TIMER: 'green' } }\n  }\n});"}

Interpreters

This feature allows you to interpret and execute the state machines and statecharts. The code sample demonstrates how to start a service that interprets the toggleMachine and logs state transitions.

{"import { interpret } from 'xstate';\n\nconst service = interpret(toggleMachine).onTransition((state) => console.log(state.value));\n\nservice.start();\nservice.send('TOGGLE');\nservice.send('TOGGLE');"}

Actors

This feature allows you to manage machine invocations as actors, which can send and receive events from other machines. The code sample shows how to spawn a child machine within a parent machine's context.

{"import { spawn, Machine } from 'xstate';\n\nconst parentMachine = Machine({\n  context: {\n    child: null\n  },\n  entry: ['spawnChild']\n}, {\n  actions: {\n    spawnChild: assign({\n      child: () => spawn(someMachine)\n    })\n  }\n});"}

Other packages similar to xstate

robot3

robot3 is a functional, immutable finite state machine library with a similar API to xstate. It is smaller in size but does not offer the full range of features such as hierarchical and parallel states.

state-machine-cat

state-machine-cat allows you to visualize state machines and statecharts. While it does not execute state machines like xstate, it is useful for documentation and analysis purposes.

redux-saga

redux-saga is a library that handles side effects in Redux applications using sagas, which are similar to state machines. It is more focused on managing side effects than modeling state, unlike xstate which is more general-purpose.

README

Actor-based state management & orchestration for complex app logic. → Documentation

XState is a state management and orchestration solution for JavaScript and TypeScript apps. It has zero dependencies, and is useful for frontend and backend application logic.

It uses event-driven programming, state machines, statecharts, and the actor model to handle complex logic in predictable, robust, and visual ways. XState provides a powerful and flexible way to manage application and workflow state by allowing developers to model logic as actors and state machines.

✨ Create state machines visually in Stately Studio → state.new

---

📖 Read the documentation

➡️ Create state machines with the Stately Editor

🖥 Download our VS Code extension

📑 Inspired by the SCXML specification

💬 Chat on the Stately Discord Community

Templates

Get started by forking one of these templates on CodeSandbox:

Template
🤖 XState Template (CodeSandbox)	XState v5 TypeScript No framework
⚛️ XState + React Template (CodeSandbox)	React XState v5 TypeScript
💚 XState + Vue Template (CodeSandbox)	Vue XState v5 TypeScript
🧡 XState + Svelte Template (CodeSandbox)	Svelte XState v5 TypeScript

Super quick start

npm install xstate

import { createMachine, createActor, assign } from 'xstate';

// State machine
const toggleMachine = createMachine({
  id: 'toggle',
  initial: 'inactive',
  context: {
    count: 0
  },
  states: {
    inactive: {
      on: {
        TOGGLE: { target: 'active' }
      }
    },
    active: {
      entry: assign({ count: ({ context }) => context.count + 1 }),
      on: {
        TOGGLE: { target: 'inactive' }
      }
    }
  }
});

// Actor (instance of the machine logic, like a store)
const toggleActor = createActor(toggleMachine);
toggleActor.subscribe((state) => console.log(state.value, state.context));
toggleActor.start();
// => logs 'inactive', { count: 0 }

toggleActor.send({ type: 'TOGGLE' });
// => logs 'active', { count: 1 }

toggleActor.send({ type: 'TOGGLE' });
// => logs 'inactive', { count: 1 }

Stately Studio

• Visually create, edit, and collaborate on state machines
• Export to many formats, including XState v5
• Test path & documentation autogeneration
• Deploy to Stately Sky
• Generate & modify machines with Stately AI

state.new

Why?

Statecharts are a formalism for modeling stateful, reactive systems. This is useful for declaratively describing the behavior of your application, from the individual components to the overall application logic.

Read 📽 the slides (🎥 video) or check out these resources for learning about the importance of finite state machines and statecharts in user interfaces:

• Statecharts - A Visual Formalism for Complex Systems by David Harel
• The World of Statecharts by Erik Mogensen

Packages

Package	Description
🤖 xstate	Core finite state machine and statecharts library + interpreter
📉 @xstate/graph	Graph traversal utilities for XState
⚛️ @xstate/react	React hooks and utilities for using XState in React applications
💚 @xstate/vue	Vue composition functions and utilities for using XState in Vue applications
🎷 @xstate/svelte	Svelte utilities for using XState in Svelte applications
🥏 @xstate/solid	Solid hooks and utilities for using XState in Solid applications
✅ @xstate/test	Model-Based-Testing utilities (using XState) for testing any software
🔍 @xstate/inspect	Inspection utilities for XState

Finite State Machines

Code

Statechart

import { createMachine, createActor } from 'xstate'; const lightMachine = createMachine({ id: 'light', initial: 'green', states: { green: { on: { TIMER: 'yellow' } }, yellow: { on: { TIMER: 'red' } }, red: { on: { TIMER: 'green' } } } }); const actor = createActor(lightMachine); actor.subscribe((state) => { console.log(state.value); }); actor.start(); // logs 'green' actor.send({ type: 'TIMER' }); // logs 'yellow' Code Statechart

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Hierarchical (Nested) State Machines

Code

Statechart

import { createMachine, createActor } from 'xstate'; const pedestrianStates = { initial: 'walk', states: { walk: { on: { PED_TIMER: 'wait' } }, wait: { on: { PED_TIMER: 'stop' } }, stop: {} } }; const lightMachine = createMachine({ id: 'light', initial: 'green', states: { green: { on: { TIMER: 'yellow' } }, yellow: { on: { TIMER: 'red' } }, red: { on: { TIMER: 'green' }, ...pedestrianStates } } }); const actor = createActor(lightMachine); actor.subscribe((state) => { console.log(state.value); }); actor.start(); // logs 'green' actor.send({ type: 'TIMER' }); // logs 'yellow' actor.send({ type: 'TIMER' }); // logs { red: 'walk' } actor.send({ type: 'PED_TIMER' }); // logs { red: 'wait' }

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Parallel State Machines

Code

Statechart

import { createMachine, createActor } from 'xstate'; const wordMachine = createMachine({ id: 'word', type: 'parallel', states: { bold: { initial: 'off', states: { on: { on: { TOGGLE_BOLD: 'off' } }, off: { on: { TOGGLE_BOLD: 'on' } } } }, underline: { initial: 'off', states: { on: { on: { TOGGLE_UNDERLINE: 'off' } }, off: { on: { TOGGLE_UNDERLINE: 'on' } } } }, italics: { initial: 'off', states: { on: { on: { TOGGLE_ITALICS: 'off' } }, off: { on: { TOGGLE_ITALICS: 'on' } } } }, list: { initial: 'none', states: { none: { on: { BULLETS: 'bullets', NUMBERS: 'numbers' } }, bullets: { on: { NONE: 'none', NUMBERS: 'numbers' } }, numbers: { on: { BULLETS: 'bullets', NONE: 'none' } } } } } }); const actor = createActor(wordMachine); actor.subscribe((state) => { console.log(state.value); }); actor.start(); // logs { // bold: 'off', // italics: 'off', // underline: 'off', // list: 'none' // } actor.send({ type: 'TOGGLE_BOLD' }); // logs { // bold: 'on', // italics: 'off', // underline: 'off', // list: 'none' // } actor.send({ type: 'TOGGLE_ITALICS' }); // logs { // bold: 'on', // italics: 'on', // underline: 'off', // list: 'none' // }

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History States

Code

Statechart

import { createMachine, createActor } from 'xstate'; const paymentMachine = createMachine({ id: 'payment', initial: 'method', states: { method: { initial: 'cash', states: { cash: { on: { SWITCH_CHECK: 'check' } }, check: { on: { SWITCH_CASH: 'cash' } }, hist: { type: 'history' } }, on: { NEXT: 'review' } }, review: { on: { PREVIOUS: 'method.hist' } } } }); const actor = createActor(paymentMachine); actor.subscribe((state) => { console.log(state.value); }); actor.start(); // logs { // value: { method: 'cash' }, // } actor.send({ type: 'SWITCH_CHECK' }); // logs { // value: { method: 'check' }, // } actor.send({ type: 'NEXT' }); // logs { // value: 'review', // } actor.send({ type: 'PREVIOUS' }); // logs { // value: { method: 'check' }, // }

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Sponsors

Special thanks to the sponsors who support this open-source project:

SemVer Policy

We understand the importance of the public contract and do not intend to release any breaking changes to the runtime API in a minor or patch release. We consider this with any changes we make to the XState libraries and aim to minimize their effects on existing users.

Breaking changes

XState executes much of the user logic itself. Therefore, almost any change to its behavior might be considered a breaking change. We recognize this as a potential problem but believe that treating every change as a breaking change is not practical. We do our best to implement new features thoughtfully to enable our users to implement their logic in a better, safer way.

Any change could affect how existing XState machines behave if those machines are using particular configurations. We do not introduce behavior changes on a whim and aim to avoid making changes that affect most existing machines. But we reserve the right to make some behavior changes in minor releases. Our best judgment of the situation will always dictate such changes. Please always read our release notes before deciding to upgrade.

TypeScript changes

We also reserve a similar right to adjust declared TypeScript definitions or drop support for older versions of TypeScript in a minor release. The TypeScript language itself evolves quickly and often introduces breaking changes in its minor releases. Our team is also continuously learning how to leverage TypeScript more effectively - and the types improve as a result.

For these reasons, it is impractical for our team to be bound by decisions taken when an older version of TypeScript was its latest version or when we didn’t know how to declare our types in a better way. We won’t introduce declaration changes often - but we are more likely to do so than with runtime changes.

Packages

Most of the packages in the XState family declare a peer dependency on XState itself. We’ll be cautious about maintaining compatibility with already-released packages when releasing a new version of XState, but each release of packages depending on XState will always adjust the declared peer dependency range to include the latest version of XState. For example, you should always be able to update xstate without @xstate/react. But when you update @xstate/react, we highly recommend updating xstate too.