Demo

		type MyStates = "disabled" | "idle" | "running";
type MyEvents = "toggleEnabled" | "start" | "stop";
const f = new FiniteStateMachine<MyStates, MyEvents>("disabled", {
	disabled: {
		toggleEnabled: "idle"
	},
	idle: {
		toggleEnabled: "disabled",
		start: "running"
	},
	running: {
		_enter: () => {
			f.debounce(2000, "stop");
		},
		stop: "idle",
		toggleEnabled: "disabled"
	}
});
	

Usage

Finite state machines (often abbreviated as "FSMs") are useful for tracking and manipulating something that could be in one of many different states. It centralizes the definition of every possible state and the events that might trigger a transition from one state to another. Here is a state machine describing a simple toggle switch:

		import { FiniteStateMachine } from "runed";
type MyStates = "on" | "off";
type MyEvents = "toggle";
 
const f = new FiniteStateMachine<MyStates, MyEvents>("off", {
	off: {
		toggle: "on"
	},
	on: {
		toggle: "off"
	}
});
	

The first argument to the FiniteStateMachine constructor is the initial state. The second argument is an object with one key for each state. Each state then describes which events are valid for that state, and which state that event should lead to.

In the above example of a simple switch, there are two states (on and off). The toggle event in either state leads to the other state.

You send events to the FSM using f.send. To send the toggle event, invoke f.send('toggle').

Actions

Maybe you want fancier logic for an event handler, or you want to conditionally transition into another state. Instead of strings, you can use actions.

An action is a function that returns a state. An action can receive parameters, and it can use those parameters to dynamically choose which state should come next. It can also prevent a state transition by returning nothing.

		type MyStates = "on" | "off" | "cooldown";
 
const f = new FiniteStateMachine<MyStates, MyEvents>("off", {
	off: {
		toggle: () => {
			if (isTuesday) {
				// Switch can only turn on during Tuesdays
				return "on";
			}
			// All other days, nothing is returned and state is unchanged.
		}
	},
	on: {
		toggle: (heldMillis: number) => {
			// You can also dynamically return the next state!
			// Only turn off if switch is depressed for 3 seconds
			if (heldMillis > 3000) {
				return "off";
			}
		}
	}
});
	

Lifecycle methods

You can define special handlers that are invoked whenever a state is entered or exited:

		const f = new FiniteStateMachine<MyStates, MyEvents>('off', {
	off: {
		toggle: 'on'
		_enter: (meta) => { console.log('switch is off') }
		_exit: (meta) => { console.log('switch is no longer off') }
	},
	on: {
		toggle: 'off'
		_enter: (meta) => { console.log('switch is on') }
		_exit: (meta) => { console.log('switch is no longer on') }
	}
});
	

The lifecycle methods are invoked with a metadata object containing some useful information:

• from: the name of the event that is being exited
• to: the name of the event that is being entered
• event: the name of the event which has triggered the transition
• args: (optional) you may pass additional metadata when invoking an action with f.send('theAction', additional, params, as, args)

The _enter handler for the initial state is called upon creation of the FSM. It is invoked with both the from and event fields set to null.

Wildcard handlers

There is one special state used as a fallback: *. If you have the fallback state, and you attempt to send() an event that is not handled by the current state, then it will try to find a handler for that event on the * state before discarding the event:

		const f = new FiniteStateMachine<MyStates, MyEvents>('off', {
	off: {
		toggle: 'on'
	},
	on: {
		toggle: 'off'
	}
	'*': {
		emergency: 'off'
	}
});
 
// will always result in the switch turning off.
f.send('emergency');
	

Debouncing

Frequently, you want to transition to another state after some time has elapsed. To do this, use the debounce method:

		f.send("toggle"); // turn on immediately
f.debounce(5000, "toggle"); // turn off in 5000 milliseconds
	

If you re-invoke debounce with the same event, it will cancel the existing timer and start the countdown over:

		// schedule a toggle in five seconds
f.debounce(5000, "toggle");
// ... less than 5000ms elapses ...
f.debounce(5000, "toggle");
// The second call cancels the original timer, and starts a new one
	

You can also use debounce in both actions and lifecycle methods. In both of the following examples, the lightswitch will turn itself off five seconds after it was turned on:

		const f = new FiniteStateMachine<MyStates, MyEvents>("off", {
	off: {
		toggle: () => {
			f.debounce(5000, "toggle");
			return "on";
		}
	},
	on: {
		toggle: "off"
	}
});
	

		const f = new FiniteStateMachine<MyStates, MyEvents>("off", {
	off: {
		toggle: "on"
	},
	on: {
		toggle: "off",
		_enter: () => {
			f.debounce(5000, "toggle");
		}
	}
});
	

Notes

FiniteStateMachine is a loving rewrite of kenkunz/svelte-fsm.

FSMs are ideal for representing many different kinds of systems and interaction patterns. FiniteStateMachine is an intentionally minimalistic implementation. If you're looking for a more powerful FSM library, statelyai/xstate is an excellent library with more features — and a steeper learning curve.