Events

What

An event is a typed message that triggers a transition. event("back")() declares one.

Declare

const back = event("back")();
const submitBasicInfo = event("submitBasicInfo")<{ email: string; name: string }>();
back.type; // "back"

Payload must be object | void. Default: no payload.

Create instances

back.create(); // { type: "back" }
submitBasicInfo.create({ email: "a@b.com", name: "A" });

Check identity

back.is(someEvent); // boolean (type guard)

Use in transitions

m.on(shippingAddress, submitShipping, (event, { context }) => {
  const s = context.get();
  s.shippingAddress = event.payload;
  context.set(s);
  return { state: payment };
});

event.payload is typed from the event ref.

Guards (conditional transitions)

No guard syntax. Plain if in the handler picks which { state } to return. Return {} to stay put.

m.on(scanning, scanDone, (event) => ({
  state: event.payload.risky ? flagged : cleared,
}));

Internal events

Declared in internal, not inputs. Emitted by effects. Processed after the current transition. Nothing external can send them via send().

const paymentOk = event("paymentOk")<{ orderId: string }>();
const paymentFail = event("paymentFail")<{ reason: string }>();

onAny

Intercepts an event in every state. State-specific handler runs first; onAny only fires if no state handler matched.

m.onAny(back, (_event, { context }) => {
  const s = checkout.state.name;
  if (s === "payment") {
    const cur = context.get();
    cur.paymentInfo = undefined;
    context.set(cur);
    return { state: shippingAddress };
  }
  if (s === "error") return { state: payment };
  return {};
});