add typestate typescript example

07-typestate-ts/main.ts

@@ -0,0 +1,30 @@
+type State = "locked" | "unlocked";
+
+class CredentialManager<S extends State> {
+  private readonly state?: S;
+
+  unlock(this: CredentialManager<"locked">, master_key: string) {
+    return this as unknown as CredentialManager<"unlocked">;
+  }
+
+  set(this: CredentialManager<"unlocked">, key: string, value: any) {
+    // ...
+  }
+
+  get(this: CredentialManager<"unlocked">, key: string): string | undefined {
+    // ...
+    return undefined;
+  }
+
+  lock(this: CredentialManager<"unlocked">): CredentialManager<"locked"> {
+    return this as unknown as CredentialManager<"locked">;
+  }
+}
+
+function main2() {
+  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();
+}