From 23481a8b523b100bbcf2ddf1be8f472c51168e0e Mon Sep 17 00:00:00 2001
From: Mohit-Srivastava-Official-Grid-Dynamics <msrivastava@griddynamics.com>
Date: Thu, 25 Jun 2026 22:01:59 +0530
Subject: [PATCH] Added TypeScript version of observer example

---
 index.ts | 218 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 218 insertions(+)
 create mode 100644 index.ts

diff --git a/index.ts b/index.ts
new file mode 100644
index 0000000..f8b8afc
--- /dev/null
+++ b/index.ts
@@ -0,0 +1,218 @@
+type TeardownLogic = () => void;
+
+interface ObserverHandlers<T> {
+  next?: (value: T) => void;
+  error?: (error: unknown) => void;
+  complete?: () => void;
+}
+
+interface Subscription {
+  unsubscribe: () => void;
+}
+
+class Observer<T> {
+  private handlers: ObserverHandlers<T>;
+  private isUnsubscribed = false;
+  private teardown?: TeardownLogic;
+
+  constructor(handlers: ObserverHandlers<T>) {
+    this.handlers = handlers;
+  }
+
+  get closed(): boolean {
+    return this.isUnsubscribed;
+  }
+
+  setTeardown(teardown?: TeardownLogic): void {
+    if (!teardown) {
+      return;
+    }
+
+    if (this.isUnsubscribed) {
+      teardown();
+      return;
+    }
+
+    this.teardown = teardown;
+  }
+
+  next(value: T): void {
+    if (this.handlers.next && !this.isUnsubscribed) {
+      this.handlers.next(value);
+    }
+  }
+
+  error(error: unknown): 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 {
+    if (this.isUnsubscribed) {
+      return;
+    }
+
+    this.isUnsubscribed = true;
+
+    if (this.teardown) {
+      this.teardown();
+      this.teardown = undefined;
+    }
+  }
+}
+
+class Observable<T> {
+  private readonly subscribeFn: (observer: Observer<T>) => TeardownLogic | void;
+
+  constructor(subscribe: (observer: Observer<T>) => TeardownLogic | void) {
+    this.subscribeFn = subscribe;
+  }
+
+  static from<T>(values: T[]): Observable<T> {
+    return new Observable<T>((observer) => {
+      for (const value of values) {
+        if (observer.closed) {
+          break;
+        }
+
+        observer.next(value);
+      }
+
+      observer.complete();
+
+      return () => {
+        console.log('unsubscribed');
+      };
+    });
+  }
+
+  subscribe(handlers: ObserverHandlers<T>): Subscription {
+    const observer = new Observer<T>(handlers);
+    const teardown = this.subscribeFn(observer);
+
+    if (teardown) {
+      observer.setTeardown(teardown);
+    }
+
+    return {
+      unsubscribe(): void {
+        observer.unsubscribe();
+      },
+    };
+  }
+}
+
+const HTTP_POST_METHOD = 'POST' as const;
+const HTTP_GET_METHOD = 'GET' as const;
+
+type HttpMethod = typeof HTTP_POST_METHOD | typeof HTTP_GET_METHOD;
+
+const HTTP_STATUS_OK = 200 as const;
+const HTTP_STATUS_INTERNAL_SERVER_ERROR = 500 as const;
+
+type HttpStatus =
+  | typeof HTTP_STATUS_OK
+  | typeof HTTP_STATUS_INTERNAL_SERVER_ERROR;
+
+type UserRole = 'user' | 'admin';
+
+interface User {
+  name: string;
+  age: number;
+  roles: UserRole[];
+  createdAt: Date;
+  isDeleted: boolean;
+}
+
+interface BaseRequest {
+  method: HttpMethod;
+  host: string;
+  path: string;
+  params: Record<string, string>;
+}
+
+interface PostRequest<TBody> extends BaseRequest {
+  method: typeof HTTP_POST_METHOD;
+  body: TBody;
+}
+
+interface GetRequest extends BaseRequest {
+  method: typeof HTTP_GET_METHOD;
+}
+
+type AppRequest = PostRequest<User> | GetRequest;
+
+interface AppResponse {
+  status: HttpStatus;
+}
+
+const userMock: User = {
+  name: 'User Name',
+  age: 26,
+  roles: ['user', 'admin'],
+  createdAt: new Date(),
+  isDeleted: false,
+};
+
+const requestsMock: AppRequest[] = [
+  {
+    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: AppRequest): AppResponse => {
+  // handling of request
+  console.log(request);
+
+  return { status: HTTP_STATUS_OK };
+};
+
+const handleError = (error: unknown): AppResponse => {
+  // handling of error
+  console.error(error);
+
+  return { status: HTTP_STATUS_INTERNAL_SERVER_ERROR };
+};
+
+const handleComplete = (): void => {
+  console.log('complete');
+};
+
+const requests$ = Observable.from<AppRequest>(requestsMock);
+
+const subscription = requests$.subscribe({
+  next: handleRequest,
+  error: handleError,
+  complete: handleComplete,
+});
+
+subscription.unsubscribe();
+
+export {};