added more docs

Author: ajay-dhangar

docs/01-getting-started/introduction.md

@@ -96,12 +96,6 @@ Ready to dive deeper? Proceed to the next page to configure your local developme
 
 ```
 docs/
-├── 03-deep-dive-core/
-│   ├── _category_.json
-│   ├── closures-and-lexical-scope.md
-│   ├── prototype-and-inheritance.md
-│   ├── execution-context-and-callstack.md
-│   └── this-keyword-explained.md
 ├── 04-asynchronous-javascript/
 │   ├── _category_.json
 │   ├── event-loop-and-task-queue.md

docs/02-javascript-fundamentals/variables-and-scope.md

@@ -53,7 +53,7 @@ Variables declared outside any function or block belong to the global scope.
 
 :::note
 
-When you run the above code, the output is undefined because the variable `globalAppTitle` is declared in the global scope and is accessible inside the function `printTitle()`.
+In the example above, `globalAppTitle` is declared in the global scope, making it accessible from anywhere in the script, including inside functions like `printTitle()`.
 
 So, when we call `printTitle()`, it logs the value of `globalAppTitle` to the console.
 
@@ -107,9 +107,9 @@ Try editing the script below to observe how block scope and variable re-assignme
 
 ## Best Practices
 
-1. **Default to `const**`: Protect variables from unintended re-assignments.
+1. **Default to `const`**: Protect variables from unintended re-assignments.
 2. **Use `let` for reassignment**: Use only when values must mutate (e.g., loop counters, accumulators).
-3. **Avoid `var**`: Prevent scope-leakage issues and silent hoisting bugs in modern applications.
+3. **Avoid `var`**: Prevent scope-leakage issues and silent hoisting bugs in modern applications.
 
 ## Knowledge Check
 

docs/04-asynchronous-javascript/event-loop-and-task-queue.md

@@ -0,0 +1,119 @@
+---
+id: event-loop-and-task-queue
+title: "The Event Loop & Task Queue Mechanics"
+sidebar_label: Event Loop & Task Queues
+sidebar_position: 1
+description: "Master the JavaScript Event Loop, asynchronous execution concurrency, Call Stack, Microtask Queue, Macrotask Queue, and microtask starvation."
+tags: [javascript, async, event-loop, microtasks, macrotasks, concurrency]
+keywords: [javascript, async, event-loop, microtasks, macrotasks, concurrency]
+---
+
+import JSEditor from "@site/src/components/js-live-code-editor";
+import CodeBlock from "@theme/CodeBlock";
+import firstExample from "!!raw-loader!../_scripts/asynchronous-javascript/event-loop-playground.js";
+import secondExample from "!!raw-loader!../_scripts/asynchronous-javascript/event-loop-exercise.js";
+
+JavaScript is single-threaded, meaning it can only execute one line of code at a time on its main thread. Despite this limitation, it handles non-blocking asynchronous operations—such as network requests, file I/O, and timers—efficiently through the **Event Loop**.
+
+## Architecture of Asynchronous Execution
+
+To handle asynchronous tasks without freezing the user interface, the JavaScript engine works alongside browser Web APIs (or Node.js C++ bindings) and two distinct execution queues.
+
+```text
+JavaScript Asynchronous Architecture
+┌─────────────────────────────────────────────────────────────┐
+│ Call Stack (Synchronous Execution LIFO)                     │
+└──────────────────────────────┬──────────────────────────────┘
+                               │ Delegation
+                               ▼
+┌─────────────────────────────────────────────────────────────┐
+│ Web APIs (Timers, Fetch HTTP, DOM Events, File Systems)     │
+└──────────────────────────────┬──────────────────────────────┘
+                               │ Callbacks Ready
+                               ▼
+┌──────────────────────────────┬──────────────────────────────┐
+│ Microtask Queue (High-Priority)│ Macrotask Queue (Task Queue)
+│ ├── Promises (.then/catch)   │ ├── setTimeout / setInterval │
+│ ├── queueMicrotask()         │ ├── setImmediate (Node.js)   │
+│ └── MutationObserver         │ └── requestAnimationFrame    │
+└──────────────┬───────────────┴──────────────┬───────────────┘
+               │                              │
+               └───────────────┬──────────────┘
+                               │ Event Loop Tick
+                               ▼
+┌─────────────────────────────────────────────────────────────┐
+│ Call Stack (Executed when Call Stack is empty)              │
+└─────────────────────────────────────────────────────────────┘
+
+```
+
+## Microtasks vs. Macrotasks (Task Priorities)
+
+The Event Loop continuously monitors the **Call Stack**. When the stack is empty, it processes pending callbacks according to a strict priority hierarchy:
+
+| Queue Type | Operations | Execution Rule / Priority |
+| --- | --- | --- |
+| **Microtask Queue** | `Promise` callbacks, `queueMicrotask()`, `process.nextTick` (Node) | **Highest Priority**: Drained **completely** until empty before any macrotask runs. |
+| **Macrotask Queue** | `setTimeout`, `setInterval`, `setImmediate`, I/O, UI Rendering | **Normal Priority**: Processes **one single task** per Event Loop tick, then checks microtasks again. |
+
+## The Event Loop Execution Algorithm
+
+During every cycle ("tick") of the Event Loop, the runtime follows these exact steps:
+
+1. **Execute Synchronous Code**: Process all frames in the Call Stack until it is completely empty.
+2. **Drain Microtask Queue**: Process every single microtask currently in the queue. If a microtask schedules another microtask, it is executed in the *same* cycle.
+3. **Render UI (Browser Only)**: Perform DOM re-paints and run `requestAnimationFrame` callbacks if needed.
+4. **Execute One Macrotask**: Dequeue and run the oldest single task from the Macrotask Queue.
+5. **Repeat**: Loop back to step 1.
+
+## Microtask Starvation
+
+Because the Event Loop must drain the **entire** Microtask Queue before yielding execution to macrotasks or UI rendering, continuously enqueuing microtasks will block the main thread indefinitely:
+
+```javascript title="Infinite Microtask Starvation Example"
+// WARNING: This recursively starves the Macrotask Queue and freezes the UI!
+function infiniteMicrotask() {
+  Promise.resolve().then(() => {
+    infiniteMicrotask(); // Enqueues another microtask infinitely
+  });
+}
+
+// infiniteMicrotask(); // Un-commenting will crash/freeze the thread!
+
+```
+
+## Interactive Playground: Tracing Execution Order
+
+Predict the log sequence of synchronous statements, `setTimeout`, and `Promise.then` callbacks:
+
+<JSEditor title="Event Loop Playground" run={true}>
+  {firstExample}
+</JSEditor>
+
+## Best Practices
+
+1. **Use Promises/Microtasks for Instant State Updates**: When you need async operations to resolve immediately before DOM repainting or downstream state changes occurs.
+2. **Offload Heavy Loops with `setTimeout`**: Yield execution back to the browser frame engine by breaking intensive computations into macrotask chunks (`setTimeout(fn, 0)`).
+3. **Avoid Infinite Microtask Chaining**: Never recursively queue microtasks (`queueMicrotask` or `.then`) without exit conditions; doing so blocks DOM rendering and input events.
+
+
+## Knowledge Check
+
+### Exercise Requirements:
+
+Determine the exact console output order for the following code snippet:
+
+<JSEditor title="Event Loop Exercise" run={true}>
+  {secondExample}
+</JSEditor>
+
+#### Explanation:
+
+1. Synchronous execution logs `A` and `F`.
+2. Microtask Queue processes `C` and `E`.
+3. Inside microtask `C`, a new macrotask (`D`) is scheduled behind `B`.
+4. Macrotask Queue runs `B` first (oldest macrotask), then runs `D`.
+
+:::success Next Up
+Now that you have mastered Event Loop ordering and task queues, proceed to **Promises and Async/Await**!
+:::

docs/04-asynchronous-javascript/fetch-api-and-ajax.md

@@ -0,0 +1,162 @@
+---
+id: fetch-api-and-ajax
+title: "Fetch API & Asynchronous HTTP"
+sidebar_label: Fetch API & AJAX
+sidebar_position: 3
+description: "Master asynchronous HTTP requests in JavaScript with the Fetch API, AbortController timeout handling, streaming responses, and proper error handling."
+tags: [javascript, async, fetch, ajax, http, abortcontroller, web-apis]
+keywords: [javascript, async, fetch, ajax, http, abortcontroller, web-apis]
+---
+
+import JSEditor from "@site/src/components/js-live-code-editor";
+import CodeBlock from "@theme/CodeBlock";
+import firstExample from "!!raw-loader!../_scripts/asynchronous-javascript/live-fetch.js";
+
+Asynchronous JavaScript and XML (**AJAX**) allows web applications to communicate with backend servers asynchronously without reloading the current page. Modern JavaScript applications utilize the native promise-based **Fetch API** for networking.
+
+## Evolution: `XMLHttpRequest` to `fetch()`
+
+Before `fetch()`, asynchronous requests relied on `XMLHttpRequest` (XHR), which required verbose, callback-driven code:
+
+```javascript title="legacy-xhr.js"
+const xhr = new XMLHttpRequest();
+xhr.open("GET", "https://api.example.com/data");
+xhr.onload = function () {
+  if (xhr.status >= 200 && xhr.status < 300) {
+    console.log(JSON.parse(xhr.responseText));
+  }
+};
+xhr.onerror = function () {
+  console.error("Network Error");
+};
+xhr.send();
+```
+
+The modern **Fetch API** streamlines networking with standard Promises and cleaner configuration objects:
+
+```javascript title="modern-fetch.js"
+// Modern Promise-based Fetch
+fetch("https://api.example.com/data")
+  .then((response) => response.json())
+  .then((data) => console.log(data))
+  .catch((error) => console.error("Fetch Error:", error));
+```
+
+## Configuring Requests & Headers
+
+The `fetch()` function accepts two arguments: the target `resource` URL and an optional `options` configuration object.
+
+```javascript title="fetch-config.js"
+async function createUser(userData) {
+  const response = await fetch("https://api.example.com/users", {
+    method: "POST", // HTTP Method: GET, POST, PUT, DELETE, PATCH
+    headers: {
+      "Content-Type": "application/json",
+      "Authorization": "Bearer YOUR_JWT_TOKEN"
+    },
+    body: JSON.stringify(userData), // Serialize JavaScript object to JSON
+    mode: "cors", // cors, no-cors, or same-origin
+    cache: "no-cache"
+  });
+
+  return await response.json();
+}
+
+```
+
+## The Fetch Error-Handling Gotcha
+
+Unlike HTTP libraries like Axios, `fetch()` **does not reject** its Promise on HTTP error status codes (such as `404 Not Found` or `500 Internal Server Error`). A `fetch()` Promise only rejects on network failures or blocked requests.
+
+To handle HTTP errors properly, check the `response.ok` boolean property (`status` between 200–299):
+
+```javascript title="safe-fetch.js"
+async function safeFetch(url) {
+  try {
+    const response = await fetch(url);
+
+    // Check for HTTP error status codes (4xx, 5xx)
+    if (!response.ok) {
+      throw new Error(`HTTP Error! Status: ${response.status} ${response.statusText}`);
+    }
+
+    const data = await response.json();
+    return data;
+  } catch (error) {
+    console.error("Request Failed:", error.message);
+  }
+}
+
+```
+
+## Aborting Requests & Timeouts (`AbortController`)
+
+To cancel ongoing network requests or implement request timeouts, use the native **`AbortController`** interface:
+
+```javascript title="abort-fetch.js"
+async function fetchWithTimeout(resource, options = {}) {
+  const { timeout = 5000 } = options;
+
+  // 1. Create an AbortController instance
+  const controller = new AbortController();
+  const id = setTimeout(() => controller.abort(), timeout);
+
+  try {
+    const response = await fetch(resource, {
+      ...options,
+      signal: controller.signal // Connect signal to fetch
+    });
+    clearTimeout(id);
+    return await response.json();
+  } catch (error) {
+    if (error.name === "AbortError") {
+      throw new Error("Request timed out!");
+    }
+    throw error;
+  }
+}
+
+```
+
+## Interactive Playground: Live Fetch Explorer
+
+Run the live `fetch()` request below using JSONPlaceholder API:
+
+<JSEditor title="Live Fetch Playground" run={true}>
+  {firstExample}
+</JSEditor>      
+
+## Best Practices
+
+1. **Always Check `response.ok`**: Never assume a resolved `fetch()` promise indicates a successful data payload.
+2. **Set Request Timeouts**: Wrap network calls with `AbortController` to prevent hanging pending requests on poor connections.
+3. **Use Typed Response Parsing**: Match body parsing methods to payloads (`.json()`, `.text()`, `.blob()`, or `.arrayBuffer()`).
+
+## Knowledge Check
+
+### Exercise Requirements:
+
+Write a reusable function `postJSON(url, payload)` using `async/await` that sends a `POST` request, handles non-2xx HTTP errors properly, and returns the parsed JSON response.
+
+```javascript title="solution.js"
+async function postJSON(url, payload) {
+  const response = await fetch(url, {
+    method: "POST",
+    headers: {
+      "Content-Type": "application/json"
+    },
+    body: JSON.stringify(payload)
+  });
+
+  if (!response.ok) {
+    throw new Error(`Failed to POST to ${url} - Status: ${response.status}`);
+  }
+
+  return await response.json();
+}
+
+```
+
+:::success Phase 04 Complete!
+Congratulations! You have completed **Phase 04: Asynchronous JavaScript**. Proceed to **Phase 05: DOM & Browser APIs** to build interactive Web applications!
+:::