XState Docs

# Actions

Actions are fire-and-forget effects. They can be declared in three ways:

  • entry actions are executed upon entering a state
  • exit actions are executed upon exiting a state
  • transition actions are executed when a transition is taken

To learn more, read about actions in our introduction to statecharts.

# API

Actions can be added like so:










 
 




 
 
 
 







 
 
 
 
 
 
 
 
 
 
 
 
 
 
 



const triggerMachine = createMachine(
  {
    id: 'trigger',
    initial: 'inactive',
    states: {
      inactive: {
        on: {
          TRIGGER: {
            target: 'active',
            // transition actions
            actions: ['activate', 'sendTelemetry']
          }
        }
      },
      active: {
        // entry actions
        entry: ['notifyActive', 'sendTelemetry'],
        // exit actions
        exit: ['notifyInactive', 'sendTelemetry'],
        on: {
          STOP: { target: 'inactive' }
        }
      }
    }
  },
  {
    actions: {
      // action implementations
      activate: (context, event) => {
        console.log('activating...');
      },
      notifyActive: (context, event) => {
        console.log('active!');
      },
      notifyInactive: (context, event) => {
        console.log('inactive!');
      },
      sendTelemetry: (context, event) => {
        console.log('time:', Date.now());
      }
    }
  }
);
When should I use transition vs. entry/exit actions?

It depends! They mean different things:

  • An entry/exit action means “execute this action on any transition that enters/exits this state”. Use entry/exit actions when the action is only dependent on the state node that it’s in, and not on previous/next state nodes or events.
// ...
{
  idle: {
    on: {
      LOAD: 'loading'
    }
  },
  loading: {
    // this action is executed whenever the 'loading' state is entered
    entry: 'fetchData'
  }
}
// ...
  • A transition action means “execute this action only on this transition”. Use transition actions when the action is dependent on the event and the state node that it is currently in.
// ...
{
  idle: {
    on: {
      LOAD: {
        target: 'loading',
        // this action is executed only on this transition
        actions: 'fetchData'
    }
  },
  loading: {
    // ...
  }
}
// ...

TIP

Action implementations can be quickly prototyped by specifying the action function directly in the machine config:




 



// ...
TRIGGER: {
  target: 'active',
  actions: (context, event) => { console.log('activating...'); }
}
// ...

Refactoring inline action implementations in the actions property of the machine options makes it easier to debug, serialize, test, and accurately visualize actions.

# Declarative actions

The State instance returned from machine.transition(...) has an .actions property, which is an array of action objects for the interpreter to execute:




 
 
 
 
 
 

const activeState = triggerMachine.transition('inactive', { type: 'TRIGGER' });

console.log(activeState.actions);
// [
//   { type: 'activate', exec: ... },
//   { type: 'sendTelemetry', exec: ... },
//   { type: 'notifyActive', exec: ... },
//   { type: 'sendTelemetry', exec: ... }
// ]

Each action object has two properties (and others, that you can specify):

  • type - the action type
  • exec - the action implementation function

The exec function takes three arguments:

Argument Type Description
context TContext The current machine context
event event object The event that caused the transition
actionMeta meta object An object containing meta data about the action (see below)

The actionMeta object includes the following properties:

Property Type Description
action action object The original action object
state State The resolved machine state, after transition

The interpreter will call the exec function with the currentState.context, the event, and the state that the machine transitioned to. You can customize this behavior. Read executing actions for more details.

# Action order

When interpreting statecharts, the order of actions should not necessarily matter (that is, they should not be dependent on each other). However, the order of the actions in the state.actions array is:

  1. exit actions - all the exit actions of the exited state node(s), from the atomic state node up
  2. transition actions - all actions defined on the chosen transition
  3. entry actions - all the entry actions of the entered state node(s), from the parent state down

WARNING

In XState version 4.x, assign actions have priority and are executed before any other actions. This behavior will be fixed in version 5, as the assign actions will be called in order.

DANGER

All action creators documented here return action objects; it is a pure function that only returns an action object and does not imperatively send an event. Do not imperatively call action creators; they will do nothing!

// 🚫 Do not do this!
entry: () => {
  // 🚫 This will do nothing; send() is not an imperative function!
  send({ type: 'SOME_EVENT' });
};

console.log(send({ type: 'SOME_EVENT' }));
// => { type: 'xstate.send', event: { type: 'SOME_EVENT' } }

// ✅ Do this instead
entry: send({ type: 'SOME_EVENT' });

# Send action

The send(event) action creator creates a special “send” action object that tells a service (i.e., interpreted machine) to send that event to itself. It queues an event to the running service, in the external event queue, which means the event is sent on the next “step” of the interpreter.

Argument Type Description
event string or event object or event expression The event to send to the specified options.to (or self)
options? send options (see below) Options for sending the event.

The send options argument is an object containing:

Property Type Description
id? string The send ID (used for cancellation)
to? string The target of the event (defaults to self)
delay? number The timeout (milliseconds) before sending the event, if the event is not canceled before the timeout

WARNING

The send(...) function is an action creator; it is a pure function that only returns an action object and does not imperatively send an event.

import { createMachine, send } from 'xstate';

const lazyStubbornMachine = createMachine({
  id: 'stubborn',
  initial: 'inactive',
  states: {
    inactive: {
      on: {
        TOGGLE: {
          target: 'active',
          // send the TOGGLE event again to the service
          actions: send('TOGGLE')
        }
      }
    },
    active: {
      on: {
        TOGGLE: { target: 'inactive' }
      }
    }
  }
});

const nextState = lazyStubbornMachine.transition('inactive', {
  type: 'TOGGLE'
});

nextState.value;
// => 'active'
nextState.actions;
// => [{ type: 'xstate.send', event: { type: 'TOGGLE' }}]

// The service will proceed to send itself the { type: 'TOGGLE' } event.

The event argument passed to send(event) can be:

  • A string event, e.g., send('TOGGLE')
  • An event object, e.g., send({ type: 'TOGGLE', payload: ... })
  • An event expression, which is a function that takes in the current context and event that triggered the send() action, and returns an event object:
import { send } from 'xstate';

// contrived example - reads from the `context` and sends
// the dynamically created event
const sendName = send((context, event) => ({
  type: 'NAME',
  name: context.user.name
}));

const machine = createMachine({
  // ...
  on: {
    TOGGLE: {
      actions: sendName
    }
  }
  //...
});

# Send targets

An event sent from a send(...) action creator can signify that it should be sent to specific targets, such as invoked services or spawned actors. This is done by specifying the { to: ... } property in the send(...) action:

// ...
invoke: {
  id: 'some-service-id',
  src: 'someService',
  // ...
},
// ...
// Indicates to send { type: 'SOME_EVENT' } to the invoked service
actions: send({ type: 'SOME_EVENT' }, { to: 'some-service-id' })

The target in the to prop can also be a target expression, which is a function that takes in the current context and event that triggered the action, and returns either a string target or an actor reference:

entry: assign({
  someActor: () => {
    return spawn(someMachine, 'some-actor-name');
  }
}),
  // ...

  // Send { type: 'SOME_EVENT' } to the actor ref
  {
    actions: send(
      { type: 'SOME_EVENT' },
      {
        to: (context) => context.someActor
      }
    )
  };

WARNING

Again, the send(...) function is an action creator and will not imperatively send an event. Instead, it returns an action object that describes where the event will be sent to:

console.log(send({ type: 'SOME_EVENT' }, { to: 'child' }));
// logs:
// {
//   type: 'xstate.send',
//   to: 'child',
//   event: {
//     type: 'SOME_EVENT'
//   }
// }

To send from a child machine to a parent machine, use sendParent(event) (takes the same arguments as send(...)).

# Raise action

The raise() action creator queues an event to the statechart, in the internal event queue. This means the event is sent immediately on the current “step” of the interpreter.

Argument Type Description
event string or event object The event to raise.
import { createMachine, actions } from 'xstate';
const { raise } = actions;

const raiseActionDemo = createMachine({
  id: 'raisedmo',
  initial: 'entry',
  states: {
    entry: {
      on: {
        STEP: {
          target: 'middle'
        },
        RAISE: {
          target: 'middle',
          // immediately invoke the NEXT event for 'middle'
          actions: raise('NEXT')
        }
      }
    },
    middle: {
      on: {
        NEXT: { target: 'last' }
      }
    },
    last: {
      on: {
        RESET: { target: 'entry' }
      }
    }
  }
});

Click on both STEP and RAISE events in the visualizer (opens new window) to see the difference.

# Respond action 4.7+

The respond() action creator creates a send() action that is sent to the service that sent the event which triggered the response.

This uses SCXML events internally to get the origin from the event and set the to of the send() action to the origin.

Argument Type Description
event string, event object, or send expression The event to send back to the sender
options? send options object Options to pass into the send() event

# Example using respond action

This demonstrates some parent service (authClientMachine) sending a 'CODE' event to the invoked authServerMachine, and the authServerMachine responding with a 'TOKEN' event.

const authServerMachine = createMachine({
  initial: 'waitingForCode',
  states: {
    waitingForCode: {
      on: {
        CODE: {
          actions: respond({ type: 'TOKEN' }, { delay: 10 })
        }
      }
    }
  }
});

const authClientMachine = createMachine({
  initial: 'idle',
  states: {
    idle: {
      on: {
        AUTH: { target: 'authorizing' }
      }
    },
    authorizing: {
      invoke: {
        id: 'auth-server',
        src: authServerMachine
      },
      entry: send('CODE', { to: 'auth-server' }),
      on: {
        TOKEN: { target: 'authorized' }
      }
    },
    authorized: {
      type: 'final'
    }
  }
});

See 📖 Sending Responses for more details.

# forwardTo action 4.7+

The forwardTo() action creator creates a send() action that forwards the most recent event to the specified service via its ID.

Argument Type Description
target string or function that returns service The target service to send the most recent event to.

# Example using forwardTo action

import { createMachine, forwardTo, interpret } from 'xstate';

function alertService(_, receive) {
  receive((event) => {
    if (event.type === 'ALERT') {
      alert(event.message);
    }
  });
}

const parentMachine = createMachine({
  id: 'parent',
  invoke: {
    id: 'alerter',
    src: () => alertService
  },
  on: {
    ALERT: { actions: forwardTo('alerter') }
  }
});

const parentService = interpret(parentMachine).start();

parentService.send({ type: 'ALERT', message: 'hello world' });
// => alerts "hello world"

# Escalate action 4.7+

The escalate() action creator escalates an error by sending it to the parent machine. This is sent as a special error event that is recognized by the machine.

Argument Type Description
errorData any The error data to escalate (send) to the parent.

# Example using escalate action

import { createMachine, actions } from 'xstate';
const { escalate } = actions;

const childMachine = createMachine({
  // ...
  // This will be sent to the parent machine that invokes this child
  entry: escalate({ message: 'This is some error' })
});

const parentMachine = createMachine({
  // ...
  invoke: {
    src: childMachine,
    onError: {
      actions: (context, event) => {
        console.log(event.data);
        //  {
        //    type: ...,
        //    data: {
        //      message: 'This is some error'
        //    }
        //  }
      }
    }
  }
});

# Log action

The log() action creator is a declarative way of logging anything related to the current state context and/or event. It takes two optional arguments:

Argument Type Description
expr? string or function A plain string or a function that takes the context and event as arguments and returns a value to be logged
label? string A string to label the logged message








 




 
 
 
 










 
 
 
 
 
 
 




import { createMachine, actions } from 'xstate';
const { log } = actions;

const loggingMachine = createMachine({
  id: 'logging',
  context: { count: 42 },
  initial: 'start',
  states: {
    start: {
      entry: log('started!'),
      on: {
        FINISH: {
          target: 'end',
          actions: log(
            (context, event) => `count: ${context.count}, event: ${event.type}`,
            'Finish label'
          )
        }
      }
    },
    end: {}
  }
});

const endState = loggingMachine.transition('start', 'FINISH');

endState.actions;
// [
//   {
//     type: 'xstate.log',
//     label: 'Finish label',
//     expr: (context, event) => ...
//   }
// ]

// The interpreter would log the action's evaluated expression
// based on the current state context and event.

Without any arguments, log() is an action that logs an object with context and event properties, containing the current context and triggering event, respectively.

# Choose action

The choose() action creator creates an action that specifies which actions should be executed based on some conditions.

Argument Type Description
conds array An array of objects containing the actions to execute when the given cond is true (see below)

Returns:

A special "xstate.choose" action object that is internally evaluated to conditionally determine which action objects should be executed.

Each "conditional actions" object in cond has these properties:

  • actions - the action objects to execute
  • cond? - the condition for executing those actions

WARNING

Do not use the choose() action creator to execute actions that can otherwise be represented as non-conditional actions executed in certain states/transitions via entry, exit, or actions.

import { actions } from 'xstate';

const { choose, log } = actions;

const maybeDoThese = choose([
  {
    cond: 'cond1',
    actions: [
      // selected when "cond1" is true
      log('cond1 chosen!')
    ]
  },
  {
    cond: 'cond2',
    actions: [
      // selected when "cond1" is false and "cond2" is true
      log((context, event) => {
        /* ... */
      }),
      log('another action')
    ]
  },
  {
    cond: (context, event) => {
      // some condition
      return false;
    },
    actions: [
      // selected when "cond1" and "cond2" are false and the inline `cond` is true
      (context, event) => {
        // some other action
      }
    ]
  },
  {
    actions: [
      log('fall-through action')
      // selected when "cond1", "cond2", and "cond3" are false
    ]
  }
]);

This is analogous to the SCXML <if>, <elseif>, and <else> elements: www.w3.org/TR/scxml/#if (opens new window)

# Pure action

The pure() action creator is a pure function (hence the name) that returns the action object(s) to be executed based on the current state context and event that triggered the action. This allows you to dynamically define which actions should be executed.

Argument Type Description
getActions function A function that returns the action object(s) to be executed based on the given context and event (see below)

Returns:

A special "xstate.pure" action object that will internally evaluate the get property to determine the action objects that should be executed.

Arguments for getActions(context, event):

Argument Type Description
context object The current state context
event event object The event object that triggered the actions

Returns:

A single action object, an array of action objects, or undefined that represents no action objects.

import { createMachine, actions } from 'xstate';

const { pure } = actions;

// Dynamically send an event to every invoked sample actor
const sendToAllSampleActors = pure((context, event) => {
  return context.sampleActors.map((sampleActor) => {
    return send('SOME_EVENT', { to: sampleActor });
  });
});
// => {
//   type: ActionTypes.Pure,
//   get: () => ... // evaluates to array of send() actions
// }

const machine = createMachine({
  // ...
  states: {
    active: {
      entry: sendToAllSampleActors
    }
  }
});

# Actions on self-transitions

A self-transition is when a state transitions to itself, in which it may exit and then reenter itself. Self-transitions can either be an internal or external transition:

  • An internal transition will not exit and reenter itself, so the state node's entry and exit actions will not be executed again.
    • Internal transitions are indicated with { internal: true }, or by leaving the target as undefined.
    • Actions defined on the transition's actions property will be executed.
  • An external transition will exit and reenter itself, so the state node's entry and exit actions will be executed again.
    • All transitions are external by default. To be explicit, you can indicate them with { internal: false }.
    • Actions defined on the transition's actions property will be executed.

For example, this counter machine has one 'counting' state with internal and external transitions:









 
 
 
 



















const counterMachine = createMachine({
  id: 'counter',
  initial: 'counting',
  states: {
    counting: {
      entry: 'enterCounting',
      exit: 'exitCounting',
      on: {
        // self-transitions
        INC: { actions: 'increment' }, // internal (implicit)
        DEC: { target: 'counting', actions: 'decrement' }, // external
        DO_NOTHING: { internal: true, actions: 'logNothing' } // internal (explicit)
      }
    }
  }
});

// External transition (exit + transition actions + entry)
const stateA = counterMachine.transition('counting', { type: 'DEC' });
stateA.actions;
// ['exitCounting', 'decrement', 'enterCounting']

// Internal transition (transition actions)
const stateB = counterMachine.transition('counting', { type: 'DO_NOTHING' });
stateB.actions;
// ['logNothing']

const stateC = counterMachine.transition('counting', { type: 'INC' });
stateB.actions;
// ['increment']
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Last Updated: 12/13/2021, 11:00:34 AM

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