Added Message Queue concept with detailed explanation, example, and exercises

message_queue.js

@@ -0,0 +1,58 @@
+/*
+📘 Concept: Message Queue (Task Queue or Callback Queue)
+--------------------------------------------------------
+The Message Queue, also known as the Task Queue or Callback Queue, works
+in conjunction with the Event Loop to handle asynchronous operations in JavaScript.
+
+When asynchronous operations like `setTimeout`, HTTP requests, or DOM events complete,
+their callback functions are placed into the Message Queue.
+
+The Event Loop continuously monitors both the Call Stack and the Message Queue.
+When the Call Stack is empty, the Event Loop takes the first callback from the
+Message Queue and pushes it onto the Call Stack for execution.
+
+This mechanism ensures that JavaScript can handle asynchronous operations
+without blocking the main thread, allowing the browser to remain responsive
+to user interactions while waiting for time-consuming operations to complete.
+
+Additionally, ES6 introduced the concept of *Microtasks* (Promise callbacks and
+async/await operations), which have higher priority than regular tasks in the
+Message Queue and are processed immediately after the current script finishes
+executing but before any tasks from the Message Queue are handled.
+*/
+
+// 🧩 Example: Message Queue vs Microtask Queue
+
+console.log("1️⃣ Start");
+
+setTimeout(() => console.log("2️⃣ setTimeout callback (Message Queue)"), 0);
+
+Promise.resolve().then(() => console.log("3️⃣ Promise callback (Microtask Queue)"));
+
+console.log("4️⃣ End");
+
+/*
+Expected Output Order:
+1️⃣ Start
+4️⃣ End
+3️⃣ Promise callback (Microtask Queue executes first)
+2️⃣ setTimeout callback (Message Queue executes after)
+*/
+
+/*
+🧠 Quiz:
+1️⃣ What role does the Message Queue play in JavaScript's concurrency model?
+2️⃣ When does the Event Loop move a callback from the Message Queue to the Call Stack?
+3️⃣ Which executes first — Microtasks or tasks in the Message Queue?
+
+💻 Exercise:
+1️⃣ Create a script that includes:
+   - Two setTimeout callbacks
+   - Two Promise callbacks
+   - A synchronous log
+
+   Try to predict and then verify the exact execution order.
+
+2️⃣ Use `queueMicrotask()` to manually add a microtask.
+   Compare its execution timing with `setTimeout()`.
+*/