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Version: XState v5

Output

Output refers to the final data that an actor produces. When an actor is responsible for performing some task, such as making a network request or running complex calculations, it will return output once it finishes that task. The output represents the result of the actor’s work. Actors only produce output when their status is “done”; i.e. when they are in their final state.

Some actors may run indefinitely and not produce any output.

Output in machine actors​

Read final states for more information on how to specify output in machine actors.

Output in promise actors​

When a promise actor resolves, it can produce output data. You can specify this output data in the .output property of the machine:

import { fromPromise, createActor } from 'xstate';

const promiseLogic = fromPromise(async () => {
  const response = await fetch('https://example.com');

  return response.json() as { message: string };
});

const actor = createActor(promiseLogic);

actor.subscribe((snapshot) => {
  if (snapshot.status === 'done') {
    console.log(snapshot.output.message);
  }
});

actor.start();

Output and TypeScript​

The output property can be typed in the .types property of the machine setup:

import { setup, createActor } from 'xstate';

const machine = setup({
  types: {
    output: {} as { total: number },
  },
}).createMachine({
  // ...
  output: ({ context }) => ({
    total: context.items.reduce((total, item) => total + item.price, 0),
  }),
});

const actor = createActor(machine);

actor.subscribe((snapshot) => {
  if (snapshot.status === 'done') {
    console.log(snapshot.output.total);
  }
});

You can read the output type from any actor using OutputFrom<typeof actor>:

import type { OutputFrom } from 'xstate';
import { machine } from './machine';

const actor = createActor(machine);

function acceptOutput<T>(output: OutputFrom<typeof actor>) {
  console.log(output.total);
}