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07-typestate-ts/README.md

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+# 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 ...`).