diff --git a/index.js b/index.js
deleted file mode 100644
index c0f3ec3..0000000
--- a/index.js
+++ /dev/null
@@ -1,127 +0,0 @@
-class Observer {
-  constructor(handlers) {
-    this.handlers = handlers;
-    this.isUnsubscribed = false;
-  }
-
-  next(value) {
-    if (this.handlers.next && !this.isUnsubscribed) {
-      this.handlers.next(value);
-    }
-  }
-
-  error(error) {
-    if (!this.isUnsubscribed) {
-      if (this.handlers.error) {
-        this.handlers.error(error);
-      }
-
-      this.unsubscribe();
-    }
-  }
-
-  complete() {
-    if (!this.isUnsubscribed) {
-      if (this.handlers.complete) {
-        this.handlers.complete();
-      }
-
-      this.unsubscribe();
-    }
-  }
-
-  unsubscribe() {
-    this.isUnsubscribed = true;
-
-    if (this._unsubscribe) {
-      this._unsubscribe();
-    }
-  }
-}
-
-class Observable {
-  constructor(subscribe) {
-    this._subscribe = subscribe;
-  }
-
-  static from(values) {
-    return new Observable((observer) => {
-      values.forEach((value) => observer.next(value));
-
-      observer.complete();
-
-      return () => {
-        console.log('unsubscribed');
-      };
-    });
-  }
-
-  subscribe(obs) {
-    const observer = new Observer(obs);
-
-    observer._unsubscribe = this._subscribe(observer);
-
-    return ({
-      unsubscribe() {
-        observer.unsubscribe();
-      }
-    });
-  }
-}
-
-const HTTP_POST_METHOD = 'POST';
-const HTTP_GET_METHOD = 'GET';
-
-const HTTP_STATUS_OK = 200;
-const HTTP_STATUS_INTERNAL_SERVER_ERROR = 500;
-
-
-const userMock = {
-  name: 'User Name',
-  age: 26,
-  roles: [
-    'user',
-    'admin'
-  ],
-  createdAt: new Date(),
-  isDeleated: false,
-};
-
-const requestsMock = [
-  {
-    method: HTTP_POST_METHOD,
-    host: 'service.example',
-    path: 'user',
-    body: userMock,
-    params: {},
-  },
-  {
-    method: HTTP_GET_METHOD,
-    host: 'service.example',
-    path: 'user',
-    params: {
-      id: '3f5h67s4s'
-    },
-  }
-];
-
-const handleRequest = (request) => {
-  // handling of request
-  return {status: HTTP_STATUS_OK};
-};
-const handleError = (error) => {
-  // handling of error
-  return {status: HTTP_STATUS_INTERNAL_SERVER_ERROR};
-};
-
-const handleComplete = () => console.log('complete');
-
-const requests$ = Observable.from(requestsMock);
-
-const subscription = requests$.subscribe({
-  next: handleRequest,
-  error: handleError,
-  complete: handleComplete
-});
-
-subscription.unsubscribe();
diff --git a/index.ts b/index.ts
new file mode 100644
index 0000000..6baec9b
--- /dev/null
+++ b/index.ts
@@ -0,0 +1,162 @@
+// Enumerators, Types & Interfaces
+
+enum HTTPMethod {
+  POST = "POST",
+  GET = "GET",
+}
+
+enum HTTPStatus {
+  OK = 200,
+  INTERNAL_SERVER_ERROR = 500,
+}
+
+type Handlers<T> = {
+  next?: (value: T) => void;
+  error?: (error: Error) => void;
+  complete?: () => void;
+};
+
+type User = {
+  name: string;
+  age: number;
+  roles: string[];
+  createdAt: Date;
+  isDeleted: boolean;
+};
+
+interface IAppRequest {
+  method: HTTPMethod;
+  host: string;
+  path: string;
+  body?: User;
+  params: Record<string, unknown>;
+}
+
+interface ISubscription {
+  unsubscribe: () => void;
+}
+
+// OBSERVER & OBSERVABLE with generics
+
+class Observer<T> {
+  private handlers: Handlers<T>;
+  private isUnsubscribed: boolean;
+  public _unsubscribe?: () => void;
+
+  constructor(handlers: Handlers<T>) {
+    this.handlers = handlers;
+    this.isUnsubscribed = false;
+  }
+
+  next(value: T): void {
+    if (this.handlers.next && !this.isUnsubscribed) {
+      this.handlers.next(value);
+    }
+  }
+
+  error(error: Error): void {
+    if (!this.isUnsubscribed) {
+      if (this.handlers.error) {
+        this.handlers.error(error);
+      }
+      this.unsubscribe();
+    }
+  }
+
+  complete(): void {
+    if (!this.isUnsubscribed) {
+      if (this.handlers.complete) {
+        this.handlers.complete();
+      }
+      this.unsubscribe();
+    }
+  }
+
+  unsubscribe(): void {
+    this.isUnsubscribed = true;
+    if (this._unsubscribe) {
+      this._unsubscribe();
+    }
+  }
+}
+
+class Observable<T> {
+  private _subscribe: (observer: Observer<T>) => () => void;
+
+  constructor(subscribe: (observer: Observer<T>) => () => void) {
+    this._subscribe = subscribe;
+  }
+
+  static from<T>(values: T[]): Observable<T> {
+    return new Observable<T>((observer) => {
+      values.forEach((value) => observer.next(value));
+      observer.complete();
+      return () => {
+        console.log("unsubscribed");
+      };
+    });
+  }
+
+  subscribe(obs: Handlers<T>): ISubscription {
+    const observer = new Observer(obs);
+    observer._unsubscribe = this._subscribe(observer);
+    return {
+      unsubscribe() {
+        observer.unsubscribe();
+      },
+    };
+  }
+}
+
+// USAGE & MOCK DATA
+
+// userMock Readonly to prevent accidental mutation.
+const userMock: Readonly<User> = {
+  name: "User Name",
+  age: 26,
+  roles: ["user", "admin"],
+  createdAt: new Date(),
+  isDeleted: false,
+};
+
+const requestsMock: IAppRequest[] = [
+  {
+    method: HTTPMethod.POST,
+    host: "service.example",
+    path: "user",
+    body: userMock,
+    params: {},
+  },
+  {
+    method: HTTPMethod.GET,
+    host: "service.example",
+    path: "user",
+    params: {
+      id: "3f5h67s4s",
+    },
+  },
+];
+
+const handleRequest = (
+  request: IAppRequest
+): { status: HTTPStatus; request: IAppRequest } => {
+  console.log(`Handling request: ${request.method} ${request.path}`);
+  return { status: HTTPStatus.OK, request };
+};
+
+const handleError = (error: Error): { status: HTTPStatus } => {
+  console.error(`Handling error: ${error.message}`);
+  return { status: HTTPStatus.INTERNAL_SERVER_ERROR };
+};
+
+const handleComplete = () => console.log("complete");
+
+const requests$: Observable<IAppRequest> = Observable.from(requestsMock); // The observable emits IAppRequest type values.
+
+const subscription: ISubscription = requests$.subscribe({
+  next: handleRequest,
+  error: handleError,
+  complete: handleComplete,
+});
+
+subscription.unsubscribe();
\ No newline at end of file