You're building a document editor. Users can navigate to older versions of a document. If they start editing an old version, it silently creates a branch in the document history — often unintentional. You need to warn them first.

Sounds simple, right? But your app has multiple ways to enter edit mode:

With traditional useState-based approaches, you'd need to find every single trigger and add a guard. Miss one? Bug. Add a new entry point later? Probably another bug.

With a state machine, editing is a state — not a boolean. Every path into that state goes through the machine. Add a guard in one place, and it applies everywhere.

Here's the mental model:

The key insight: we didn't touch any UI code that triggers edit.start. The machine routes the transition based on context.

Without a state machine, you'd end up with something like this scattered across your components:

// ❌ Every edit trigger needs the guard
function onTitleClick() {
  if (!isLatest) {
    setShowConfirmDialog(true)
    return
  }
  setIsEditing(true)
}

function onEditorClick() {
  if (!isLatest) {
    setShowConfirmDialog(true) // duplicated!
    return
  }
  setIsEditing(true)
}

// And what about auto-edit when a draft exists?
useEffect(() => {
  if (hasDraft && !isLatest) {
    // Wait, do we show the dialog here too?
    // What if the user navigates to latest while the dialog is open?
  }
}, [hasDraft, isLatest])

Every new entry point means another place to remember the guard. Every edge case (draft exists + old version + account switch) multiplies the conditionals.

With XState, the guard lives in the machine definition:

// ✅ One place, all entry points covered
'edit.start': [
  {
    target: 'confirmingOldVersionEdit',
    guard: 'canEditOldVersion',     // old version → confirm first
  },
  {
    target: 'editing',
    guard: 'canTransitionToEditing', // latest version → go ahead
  },
],

The auto-edit transition for existing drafts gets the same treatment:

// Auto-edit guard: only on latest version
always: {
  target: 'editing',
  guard: ({context}) => context.shouldAutoEdit && context.isLatestVersion,
  actions: ['clearShouldAutoEdit', 'setDepsFromPublished'],
},

The confirmation state handles only its own concern:

confirmingOldVersionEdit: {
  on: {
    'edit.confirm': {
      target: 'editing',
      actions: ['setDepsFromPublished'],
    },
    'edit.cancel': {
      target: 'loaded',
    },
  },
},

The dialog component is purely declarative — it reads state and sends events:

function OldVersionEditDialog() {
  const isConfirming = useDocumentSelector(selectIsConfirmingOldVersionEdit)
  const send = useDocumentSend()

  return (
    <AlertDialog open={isConfirming}>
      <AlertDialogContent>
        <AlertDialogTitle>Edit older version?</AlertDialogTitle>
        <AlertDialogDescription>
          Editing this version will create a new branch in the document history.
        </AlertDialogDescription>
        <AlertDialogCancel onClick={() => send({type: 'edit.cancel'})}>
          Cancel
        </AlertDialogCancel>
        <AlertDialogAction onClick={() => send({type: 'edit.confirm'})}>
          Edit Anyway
        </AlertDialogAction>
      </AlertDialogContent>
    </AlertDialog>
  )
}

Drop this component anywhere in the tree. It works because it talks to the machine, not to local state.

Here's the complete document lifecycle with the new confirmation guard:

ConcernBoolean approachState machineNew edit entry pointMust add guard manuallyAutomatic — goes through machineEdge case (draft + old version)Another if somewhereGuard on always transitionTestingMock click handlers, check booleansSend events, assert statesVisualizing behaviorRead code, build mental modelGenerate diagram from definitionAdding new states (e.g. "merging")Refactor everythingAdd a state node

The real win isn't fewer lines of code — it's fewer places where things can go wrong. When behavior lives in the machine, you can reason about it in one place, test it in isolation, and visualize it as a diagram.

State machine tests are clean because they're just input → output:

test('edit.start on old version → confirmation state', () => {
  const actor = createActor(documentMachine, {
    input: { isLatest: false, canEdit: true, /* ... */ }
  })
  actor.start()

  // Simulate document load
  actor.send({type: 'document.loaded', document: mockDoc})
  actor.send({type: 'draft.resolved', draftId: null, content: null})

  // Try to edit
  actor.send({type: 'edit.start'})

  // Machine routes to confirmation, not editing
  expect(actor.getSnapshot().matches('confirmingOldVersionEdit')).toBe(true)
})

test('edit.confirm → enters editing', () => {
  // ... setup in confirmingOldVersionEdit state ...
  actor.send({type: 'edit.confirm'})
  expect(actor.getSnapshot().matches('editing')).toBe(true)
})

No DOM, no mocking click handlers, no rendering components. Pure logic.

If you're new to state machines in UI:

The code from this post is from a real production app. You can see the full implementation in PR #459, specifically: