What are state machines and when do they help?

Question

Accepted Answer

A **state machine** models a system as a finite set of **states** plus the allowed **transitions** between them. Instead of juggling independent boolean flags, you define exactly which states exist and which transitions are legal — making impossible states unrepresentable.

## The problem: boolean soup

```js
// ❌ multiple booleans → 2^n combinations, many of them invalid
const [isLoading, setLoading] = useState(false);
const [isError, setError] = useState(false);
const [isSuccess, setSuccess] = useState(false);
// { isLoading: true, isError: true, isSuccess: true } — nonsense, but POSSIBLE
```

With independent flags, nothing stops contradictory combinations, and you must manually keep them consistent on every transition — a bug magnet.

## The state-machine model

```text
States:      idle → loading → success
                       ↓
                     error → (retry) → loading

Only defined transitions are allowed; you can't be in two states at once.
```

```js
const machine = {
  idle:    { FETCH: "loading" },
  loading: { SUCCESS: "success", ERROR: "error" },
  success: { FETCH: "loading" },
  error:   { RETRY: "loading" },
};
function transition(current, event) {
  return machine[current][event] ?? current; // only legal transitions happen
}
```

A single `state` variable holds the current state; an `event` triggers a *defined* transition. Illegal transitions (e.g. SUCCESS while idle) simply can't occur — the impossible states are gone by design.

## Real-world tooling: XState

```js
// libraries like XState formalize this with guards, actions, nested/parallel states
const toggleMachine = createMachine({
  initial: "inactive",
  states: {
    inactive: { on: { TOGGLE: "active" } },
    active:   { on: { TOGGLE: "inactive" } },
  },
});
```

## When state machines help

```text
✓ Complex flows with many states: checkout, multi-step forms/wizards, media players
✓ Authentication flows (logged out → logging in → logged in → refreshing)
✓ Anything where invalid state combinations cause bugs
✗ Overkill for simple toggles or trivial state
```

## Why it matters

State machines bring rigor to complex stateful logic by making **invalid states impossible** and transitions **explicit** — replacing fragile combinations of booleans with a clear, exhaustive model.

They're especially valuable for intricate flows (wizards, payments, auth, media) where boolean soup leads to subtle, hard-to-reproduce bugs.

Even informally (a `status` enum) the idea improves async state; formally (XState) it provides visualizable, testable, robust logic.

Knowing when to reach for them is a mark of mature state design.