design patterns learning continued

Author: mrinaljain

D/design_patterns/README.md

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+🧩 Types of Design Patterns:
+Creational — Object kaise banaye (e.g., Singleton, Factory)
+
+Structural — Objects ko efficiently kaise connect karein (e.g., Adapter, Decorator)
+
+Behavioral — Object ka behavior kaise manage karein (e.g., Observer, Strategy)

D/design_patterns/behavioural/README.md

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+# Behavioural Design Paterns
+
+Yeh patterns batate hain ki objects aapas mein kaise interact karte hain — like communication rules.
+

D/design_patterns/behavioural/observer/README.md

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+# Observer Patern (pub - sub model)
+
+Ek object mein jab koi change aaye, toh automatically dusre dependent objects ko notify karna.
+
+“Observer pattern ek publish-subscribe (pub-sub) mechanism hai jisme ek subject (observable) apne observers (subscribers) ko notify karta hai jab bhi koi change hota hai.”
+
+
+- **Subject** (Obesrvable) isko observe kiya jata hai / isko observer krte hai 
+
+- **Observer** (Subscriber) ye log observe krte hai  
+
+
+Example : Notification delivery on new video update on Youtube
+
+
+## STEPS to Create Observer 
+//1. Observwble classbanaao
+//2. constructor mai ek observer space/array bnaao.
+// 3. subscribe , unsubscribe, notify y 3 method do 
+// 4. subscribe matlab jo bhi fumction mila usko observer mai push kr 
+// 5. unsubscribe matlab observr mai ye vo fun pop
+// 6. notify matlab observer k andar k sare fun ko line se execute kr

D/design_patterns/behavioural/observer/observer.js

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+// STEPS to Create Observer 
+//1. Observwble classbanaao
+//2. constructor mai ek observer space/array bnaao.
+// 3. subscribe , unsubscribe, notify y 3 method do 
+// 4. subscribe matlab jo bhi fumction mila usko observer mai push kr 
+// 5. unsubscribe matlab observr mai ye vo fun pop
+// 6. notify matlab observer k andar k sare fun ko line se execute kr
+
+class Observable {
+  constructor() {
+    this.observers = [];
+  }
+
+  subscribe(fn) {
+    this.observers.push(fn);
+  }
+
+  unsubscribe(fn) {
+    this.observers = this.observers.filter((observer) => observer !== fn);
+  }
+
+  notify(data) {
+    this.observers.forEach((observer) => observer(data));
+  }
+}
+
+// USAGE
+const newsAgency = new Observable();
+
+function subscriber1(news) {
+  console.log("Subscriber 1 received:", news);
+}
+
+function subscriber2(news) {
+  console.log("Subscriber 2 received:", news);
+}
+
+newsAgency.subscribe(subscriber1);
+newsAgency.subscribe(subscriber2);
+
+newsAgency.notify("🔥 New Article on Design Patterns!");

D/design_patterns/builder/README.md

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+# Builder / Creational
+
+Yeh patterns kaam aate hain jab object creation ko flexible aur controlled banana ho.
+

D/design_patterns/builder/factory/README.js

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+class Car {
+  constructor(wheels, isEv, topSpeed) {
+    this.wheels = wheels;
+    this.isEv = isEv;
+    this.topSpeed = topSpeed;
+  }
+}
+
+class Bike {
+  constructor(wheels, isEv, topSpeed) {
+    this.wheels = wheels;
+    this.isEv = isEv;
+    this.topSpeed = topSpeed;
+  }
+}
+
+class VehicleFactory {
+  static createVehicle(type, config = {}) {
+    switch (type) {
+      case "car":
+        return new Car(config.wheels ?? 4, true, 250);
+      case "bike":
+        return new Bike(2, false, 180);
+      default:
+        new Error("type not found");
+    }
+  }
+}
+
+let car1 = VehicleFactory.createVehicle("car", {"wheels":8});
+
+console.log(car1);

D/design_patterns/builder/factory/factory.js

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+

D/design_patterns/builder/singleton/README.md

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+class Singleton {
+  constructor() {
+   // already instance hai to vahi return kar do 
+     if (Singleton._instance) {
+       return Singleton._instance; 
+     }
+     // common prop across all the objects created
+    this.count = 0;
+    // instance nhi hai to instance k value current object set krdo .
+   Singleton._instance = this;
+
+   // agar constructor mai return statemant nhi likha hai to vo current object(Singleton) return kr dega
+  }
+
+  increaseCount() {
+    this.count++;
+    console.log("Count:", this.count);
+  }
+}
+
+let p1 = new Singleton();
+let p2 = new Singleton();
+
+
+p1.increaseCount();
+p2.increaseCount();

D/design_patterns/builder/singleton/index.js

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+# Structural
+
+Inka kaam hota hai objects ke beech ka structure manage karna ya simplify karna.
+

D/design_patterns/builder/singleton/singleton.js

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+
+//? 1. Normal break statement
+
+for (let i = 0; i < 10; i++) {
+  if (i === 5) break;
+  console.log(i);
+}
+
+
+//? 2. Using Return statement in loop
+
+for (let item of arr) {
+  if (item === val) return "Found!";
+}
+
+//? 3. Using labels
+
+outerLoop: for (let i = 0; i < 3; i++) {
+  for (let j = 0; j < 3; j++) {
+    if (i === 1 && j === 1) break outerLoop;
+    console.log(`i=${i}, j=${j}`);
+  }
+}
+
+// ? 4. throw Exception
+
+try {
+  for (let i = 0; i < 10; i++) {
+    if (i === 5) throw new Error("Abort loop");
+    console.log(i);
+  }
+} catch (e) {
+  console.log("Loop aborted");
+}

D/design_patterns/structural/README.md

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+let amount = 23482719.98;
+
+const formater = new Intl.NumberFormat("hin-IN", {
+  style: "currency",
+  currency: "INR",
+});
+
+
+let formatedAmount = formater.format(amount);
+
+console.log(formatedAmount);

F/for/for-loop.js

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+# Server Sent Events
+
+
+Socho ek radio station hai (server), aur kai listeners hain (clients/browsers).
+Listeners apna radio on karte hain aur station se awaaz (updates) sunte rehte hain.
+Radio station baar-baar poocha nahi jaata, ki "kuch naya hai kya?"
+Bas jab kuch naya aata hai, radio khud bol deta hai — aur listeners sun lete hain.
+
+Yeh hi concept hai SSE ka.

F/formater/formater.js

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+app.get("/endpoint", (req, res)=>{
+
+   // this 3 header are most important to keep the stream alive 
+res.header = {'Content-Type': 'text/event-stream'}
+res.header = {'Cache-Control': 'no-cache'}
+res.header = {'Connection': 'keep-alive'}
+
+
+// now we will send data at regular interval
+
+let intervalId = setInterval(() => {
+  res.send("updated data sent every 5 seconds");
+}, 5000);
+
+// frontend se close call hone pr ye call hoga
+req.on("close", () => {
+  clearInterval(intervalId);
+});
+
+
+});

S/server-sent-events/README.md

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+let source = new EventSource("https://sse.dev/test?interval=1");
+
+
+source.onmessage = ( message)=>{
+
+   console.log(message.data);
+   
+}
+
+// this will close the server connection 
+setTimeout(() => {
+   source.close();
+}, 10000);

S/server-sent-events/sse_backend.js

@@ -5,3 +5,8 @@ console.log(...args);
 
 
 setTimeout(callback, 5000, "new");
+
+
+setTimeout(() => {
+  console.log("Hello");
+}, Infinity);

S/server-sent-events/sse_frontend.js

@@ -11,7 +11,7 @@
 
    <div id="demo"></div>
 
-   <script src="./A/array/array.js"> </script>
+   <script src="./D/design_patterns/behavioural/observer/observer.js"> </script>
 </body>
 
 </html>