The simplest state machine in JS. You don't always need bulletproof solutions like XState. Copied from a Kent C. Dodds' article

create-simple-state-machine.js

// Copied from this article https://kentcdodds.com/blog/implementing-a-simple-state-machine-library-in-javascript
function createMachine(stateMachineDefinition) {
	const machine = {
		value: stateMachineDefinition.initialState,
		transition(currentState, event) {
			const currentStateDefinition = stateMachineDefinition[currentState]
			const destinationTransition = currentStateDefinition.transitions[event]
			if (!destinationTransition) {
				return
			}
			const destinationState = destinationTransition.target
			const destinationStateDefinition =
				stateMachineDefinition[destinationState]

			destinationTransition.action()
			currentStateDefinition.actions.onExit()
			destinationStateDefinition.actions.onEnter()

			machine.value = destinationState

			return machine.value
		},
	}
	return machine
}

const machine = createMachine({
	initialState: 'off',
	off: {
		actions: {
			onEnter() {
				console.log('off: onEnter')
			},
			onExit() {
				console.log('off: onExit')
			},
		},
		transitions: {
			switch: {
				target: 'on',
				action() {
					console.log('transition action for "switch" in "off" state')
				},
			},
		},
	},
	on: {
		actions: {
			onEnter() {
				console.log('on: onEnter')
			},
			onExit() {
				console.log('on: onExit')
			},
		},
		transitions: {
			switch: {
				target: 'off',
				action() {
					console.log('transition action for "switch" in "on" state')
				},
			},
		},
	},
})

let state = machine.value
console.log(`current state: ${state}`)
state = machine.transition(state, 'switch')
console.log(`current state: ${state}`)
state = machine.transition(state, 'switch')
console.log(`current state: ${state}`)