# Typestate Pattern in TypeScript

TypeScript allows for a very elegant implementation of the typestate pattern using `this` type parameters. This enables us to restrict method calls based on the generic state of the class instance.

```typescript
let pw_locked = new CredentialManager<"locked">();
let pw_unlocked = pw_locked.unlock("psst");
pw_unlocked.set("foo", "bar");
let val = pw_unlocked.get("foo");
pw_locked = pw_unlocked.lock();
```

By using `this: CredentialManager<"unlocked">` in the method signature, we tell TypeScript that the method can only be called when the instance is of that specific type.

```typescript
let pw_locked = new CredentialManager<"locked">();
pw_locked.set("foo", "bar"); // -> 💥 Compiler Error
```

This produces a compiler error similar to:
```
Property 'set' does not exist on type 'CredentialManager<"locked">'.
```

## Caveats

Like Go, TypeScript does not have an ownership model, so the original variable remains usable after a transition. Furthermore, TypeScript's type system is structural and can be bypassed with type assertions (`as any` or `as unknown as ...`).