Day 6 of JavaScript Mastery Journey

Welcome to Day 6 of my JavaScript Mastery Journey! 🚀✨

Author: Ravindranath Porandla ✍️

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Day 5: Learnings Are... 🚀✨

Here’s a quick recap of our adventures:

• The Mysterious this Keyword: We discovered how this is like a chameleon, changing who it points to based on how and where a function is called – whether it's the global window 🌐, an object 🏠, or inherited from a parent in arrow functions ➡️.

• Borrowing Powers (call(), apply(), bind()): We learned how to make functions temporarily "borrow" another object's identity! call() and apply() execute immediately (with arguments passed individually or in an array 💥), while bind() returns a new function with this permanently fixed for later use 🐶🔗.

• Prototypes & Prototypal Inheritance: We unlocked the secret of how objects share abilities through a "prototype chain," like a student asking their teacher for answers 🧑‍🏫. We saw how objects can "inherit" skills from others without copying them, saving memory! 💡🔄

• Pure vs. Impure Functions: We explored the difference between "clean" functions that always give the same output for the same input and don't cause side effects (like a predictable juice machine! 🧃✅) and "messy" ones that can change things outside themselves 🗑️❌.

• Arrow Functions: Our new favorite shortcut for writing functions! 🎯🛣️ We learned their concise syntax, but most importantly, how they handle this like a copycat 🐒, inheriting it from their surroundings – which is super handy for callbacks! ✨

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Alright, coding champions! 🚀 Welcome to Day 6 of our JavaScript adventure! Today, we're tackling one of the most exciting and crucial topics in modern JavaScript: Asynchronous Programming. It's all about making your code smart enough to handle tasks that take time without freezing everything up. Get ready for some mind-bending but super useful concepts! 🤯✨

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Day 6: Mastering Asynchronous JavaScript - The Art of Multitasking! 🎨⏳

Have you ever tried to do two things at once? Like listening to music while doing your homework? That's a bit like what we'll explore today: how JavaScript handles things that don't happen instantly, keeping your programs smooth and responsive! Let's dive in! 🏊‍♀️

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1. Sync vs. Async: One Thing at a Time, or Juggling Tasks? 🤹‍♀️⏱️

Imagine you have a list of chores to do. How do you tackle them?

• Synchronous (Sync) Code: The "One-After-Another" Way 🚶‍♀️🚶‍♂️

  • Analogy: It's like cooking dinner with only one cooking pot on one burner. You put the rice in, wait for it to cook completely. Only when the rice is done, you move the pot, put pasta in, wait for it to cook. And so on. You do tasks strictly one after the other.

  • How it Works: In synchronous code, each task must complete before the next task can even begin. If a task takes a long time, everything else stops and waits. It's blocking.

  • Example:

      console.log("1. Get the pot."); // This runs first
      console.log("2. Cook the rice."); // This runs after step 1 finishes
      console.log("3. Prepare the salad."); // This runs after step 2 finishes
      // Output: 1, 2, 3 in order, no waiting between lines.
    

• Asynchronous (Async) Code: The "Juggling" Way 🤸‍♂️🍳

  • Analogy: It's like cooking dinner with multiple pots on different burners, and maybe even something baking in the oven! You put the rice on one burner, then immediately start chopping veggies for the salad, then put the pasta on another burner. You're doing multiple things at the same time (or rather, managing multiple things at once). You don't wait for one thing to finish before starting another. It's non-blocking.

  • How it Works: Asynchronous code allows tasks to start, and then your program can move on to other things without waiting for the first task to finish. When the long-running task is finally done, it will notify your program.

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2. How to Spot Async Code? Look for the "Future Delivery" Notice! 📮🔮

How do you know if a piece of JavaScript code is going to be synchronous or asynchronous? Look for the special "future delivery" mechanisms!

• Common JavaScript Async Signals:

  • setTimeout() / setInterval(): These are like setting a timer for a task to happen later. ⏱️

  • Promises: These are like a promise for a future result (e.g., "I promise to get you the data!"). 🤝

  • fetch() / axios: Used for asking other computers (servers) for information over the internet. This definitely takes time! 🌐

  • XMLHttpRequest (XHR): An older way to make network requests, also asynchronous. 📞

• Example (A "Future Delivery" - setTimeout):

    console.log("Going to the kitchen now..."); // This runs first (sync)
  
    setTimeout(function() { // This entire block is an async task
      console.log("Oh, the oven timer just rang! Dinner is ready!"); // This runs AFTER 3 seconds
    }, 3000); // Wait for 3000 milliseconds (3 seconds)
  
    console.log("While waiting, I'll set the table."); // This runs immediately after the timer is set (sync)
  

  💬 Output:

    Going to the kitchen now...
    While waiting, I'll set the table.
    Oh, the oven timer just rang! Dinner is ready! (This appears after 3 seconds)
  

  See? JavaScript didn't freeze and wait for 3 seconds. It set the timer and immediately moved on! ✨

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3. Why Do We Need Async JS? When the Answer Isn't Instant! 📡❓

Imagine you're trying to find out the score of a cricket match from a distant stadium 🏏. You don't know exactly when the score will be updated and sent to you. This is exactly why we need Asynchronous JavaScript!

• The Problem: Sometimes, your JavaScript code needs to get information from somewhere else that takes time. This could be:

  • A faraway server on the internet: Like fetching new posts for your blog or checking weather. ☁️

  • A timer: Waiting for 5 seconds before showing a message. ⏳

  • A user action: Waiting for a button click or a key press. 🖱️

• The Solution: Async JS! Instead of stopping your entire program and waiting (which would make your webpage freeze!), asynchronous code lets you say: "Start this long task, but don't wait for it. I'll do other things, and you can tell me when you're done!"

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4. Callbacks & setTimeout(): "Call Me Back When You're Ready!" 📞📝

You've already seen setTimeout(), which is a classic example of an asynchronous function using a callback.

• What is a Callback?

  • Interview Definition: A callback function is a function that is passed as an argument to another function, with the expectation that the inner function will be executed later (or "called back") when a specific event or task completes.

  • Analogy: It's like leaving a sticky note with instructions for someone. You say, "Hey, when you finish building this LEGO tower, please call me back (run this function on the sticky note) to tell me it's done!" 📝

• setTimeout() and Callbacks:

  • setTimeout() is an asynchronous function provided by the browser (or Node.js environment). It takes two main ingredients:

    1. A callback function (the function() { ... } part).

    2. A delay time in milliseconds.

    function sayHelloLater() { // This is our callback function
      console.log("Hello after 2 seconds!");
    }

    setTimeout(sayHelloLater, 2000); // Pass 'sayHelloLater' as the callback
    // Or directly pass an anonymous function:
    setTimeout(function() {
      console.log("Hello after another 3 seconds!");
    }, 3000);

    console.log("I'm doing other things while waiting!");

💬 Output:

    I'm doing other things while waiting!
    Hello after 2 seconds! (appears after 2 seconds)
    Hello after another 3 seconds! (appears after 3 seconds)

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5. The Big Secret: JavaScript is NOT Asynchronous... but It Can Be! 🤫🤯

This is a super important point and a common 🌟 Interview Question! 🌟

• The Myth: People often say "JavaScript is asynchronous."

• The Truth: JavaScript itself is single-threaded. This means it has only one "main thread" (like having only one worker in a factory 👷). It can only do one thing at a time in that main thread. It's not like a multi-lane highway where many cars drive at once (that's parallelism).

• So, How Does Async Work? The Event Loop! 🔄 This is where the magic happens! JavaScript relies on its environment (like the browser or Node.js) to help with asynchronous tasks.

  1. The Call Stack (Main Stack): This is where all your synchronous JavaScript code runs, one line at a time. It's like your main "to-do" list where tasks are processed. 📜

  2. Web APIs / Node.js APIs (Side Stack/Helpers): These are like JavaScript's "helpers" provided by the browser (or Node.js). When JavaScript encounters an asynchronous task (like setTimeout, fetch), it hands that task over to these helpers. 🤝

  3. The Callback Queue (Task Queue): When an asynchronous helper finishes its job (e.g., setTimeout timer runs out, fetch gets data), it places its callback function (the code that should run next) into a waiting line called the Callback Queue. 🚦

  4. The Event Loop (The Manager!): This is the most important part! The Event Loop is constantly checking two things:

     • Is the Call Stack empty? (Is the main JavaScript worker free?)

     • Is there anything waiting in the Callback Queue?

     • If the Call Stack is empty, the Event Loop takes the very first callback from the Callback Queue and moves it onto the Call Stack to be executed!

• Analogy: The Busy Chef and the Waiter! 🧑‍🍳🍽️

  • Chef (JavaScript Engine): This is your main JavaScript thread. The chef can only cook one dish at a time.

  • Kitchen Order List (Call Stack): This is where all the chef's current cooking steps are listed.

  • Oven Timer / Delivery Service (Web APIs / Node.js APIs): These are like helpers. When the chef puts something in the oven or calls for a delivery, they hand that task to the timer or delivery service. The chef doesn't wait; they go back to the order list! 💨

  • "Dishes Ready" Counter (Callback Queue): When the oven timer rings or the delivery person arrives, they put the ready dish on this counter, waiting for the chef. 🔔

  • Restaurant Manager (Event Loop): The manager constantly checks: "Is the chef free? Is there a ready dish on the counter?" If the chef is free, the manager takes the next ready dish from the counter and gives it to the chef to plate and serve. ➡️🍽️

• Interview Question: Explain the Event Loop in JavaScript.

  • Answer: "The Event Loop is a crucial part of JavaScript's concurrency model, enabling asynchronous execution despite JavaScript being single-threaded. It constantly monitors two things: the Call Stack (where synchronous code executes) and the Callback Queue (where completed asynchronous operations place their callback functions). When the Call Stack is empty, the Event Loop takes the first function from the Callback Queue and pushes it onto the Call Stack for execution. This mechanism ensures that non-blocking operations are handled efficiently without freezing the main thread."

• Amazing Example: setTimeout(0)! 🤔 This example perfectly shows the Event Loop at work:

    console.log("Hi 1!"); // Sync code
    console.log("Hi 2!"); // Sync code
  
    setTimeout(function() { // Async code - goes to Web APIs
      console.log("Hi 3! (I waited 0ms!)");
    }, 0); // This means "put me in the queue as soon as possible!"
  
    console.log("Hi 4!"); // Sync code
  

  💬 Output:

    Hi 1!
    Hi 2!
    Hi 4!
    Hi 3! (I waited 0ms!) // This appears LAST, even with 0ms delay!
  

  Why "Hi 3!" is last:

  1. console.log("Hi 1!") runs and finishes immediately (Call Stack).

  2. console.log("Hi 2!") runs and finishes immediately (Call Stack).

  3. setTimeout() is seen. JavaScript hands the function() { console.log("Hi 3!"); } to the Web APIs (timer). The timer immediately puts the callback into the Callback Queue because the delay is 0ms. BUT, it's still in the queue, not on the Call Stack.

  4. console.log("Hi 4!") runs and finishes immediately (Call Stack).

  5. Now, the Call Stack is empty! The Event Loop sees that the Call Stack is clear and there's something in the Callback Queue (Hi 3!'s function).

  6. The Event Loop moves Hi 3!'s function from the Callback Queue to the Call Stack.

  7. Hi 3!'s function executes.

This is a fundamental concept for understanding async JavaScript! 🧠

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6. From Callbacks to Promises: Managing Async Results! 📦➡️✨

Once fetch, axios, setTimeout, setInterval, or any other async operation finishes its work, how do we get their results back? And how do we manage them gracefully? This is where Callbacks, Promises, and async/await come in!

6.1 Callbacks Revisited: "Callback Hell" Problem 😱🔥

While callbacks are essential, using many nested callbacks for sequential async operations can lead to something called "Callback Hell" or the "Pyramid of Doom." It looks like deeply indented, hard-to-read code.

• Example of Callback Hell:

    function fetchUserData(userId, callback) {
      setTimeout(() => { // Imagine this is a network request
        const userData = { id: userId, name: "Alice" };
        console.log("User data fetched!");
        callback(userData);
      }, 1000);
    }
  
    function fetchPosts(user, callback) {
      setTimeout(() => { // Another network request
        const posts = [`Post 1 by ${user.name}`, `Post 2 by ${user.name}`];
        console.log("Posts fetched!");
        callback(posts);
      }, 1500);
    }
  
    // Callback Hell:
    fetchUserData(123, function (user) {
      fetchPosts(user, function (posts) {
        console.log("All data collected!");
        posts.forEach(post => console.log(post));
        // What if we needed another step here? It would go even deeper! 😩
      });
    });
  

  This becomes very difficult to read and manage as more steps are added! 😖

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7. Promises: The "Future Result" Object! 🤝🔮

Promises were introduced to solve Callback Hell! A Promise is like a special JavaScript object that represents the eventual completion (or failure) of an asynchronous operation and its resulting value.

• Interview Definition: A Promise is an object representing the eventual completion or failure of an asynchronous operation. It acts as a placeholder for a value that will be available at some point in the future.

• Analogy: The Pizza Delivery Order! 🍕

  • You order a pizza 🍕. You don't get the pizza instantly.

  • The pizza shop gives you a Promise (your order receipt).

  • States of a Promise:

    1. Pending: You've ordered, but the pizza isn't ready yet. (Waiting for delivery). ⏳

    2. Fulfilled (or Resolved): The pizza arrives, hot and delicious! (The operation succeeded). ✅

    3. Rejected: The delivery driver got lost, or the oven broke down! (The operation failed). ❌

• Using Promises: .then(), .catch(), .finally()

  • .then(): What to do when the Promise is Fulfilled (Resolved). (When the pizza arrives!)

  • .catch(): What to do when the Promise is Rejected. (When the pizza order fails!)

  • .finally(): What to do regardless of whether the Promise succeeded or failed. (Clean up the kitchen, whether pizza came or not!)

• Example (Creating and Consuming a Promise):

    // 1. Creating a Promise (like the pizza shop promising a pizza)
    const orderPizza = new Promise((resolve, reject) => {
      const isPizzaReady = true; // Let's say the pizza is ready!
  
      setTimeout(() => { // Simulate pizza cooking time
        if (isPizzaReady) {
          resolve("Your delicious pizza is here! 🍕"); // Success!
        } else {
          reject("Oh no, pizza oven broke! 😭"); // Failure!
        }
      }, 2000);
    });
  
    // 2. Consuming the Promise (like waiting for your pizza)
    console.log("Ordering pizza now...");
    orderPizza
      .then((message) => { // This runs if 'resolve' is called
        console.log("🥳 Success! " + message);
      })
      .catch((error) => { // This runs if 'reject' is called
        console.error("😢 Error: " + error);
      })
      .finally(() => { // This runs always, whether success or error
        console.log("Finished waiting for pizza, ready for anything!");
      });
  
    console.log("While waiting, I'll watch some TV.");
  

  💬 Output:

    Ordering pizza now...
    While waiting, I'll watch some TV.
    🥳 Success! Your delicious pizza is here! 🍕 (appears after 2 seconds)
    Finished waiting for pizza, ready for anything!
  

• Promise Chaining (Solving Callback Hell!): You can link .then() calls together to handle a sequence of asynchronous operations, which makes the code much flatter and easier to read than nested callbacks. This is called Promise Chaining.

    function deliverPizza() {
      return new Promise(resolve => {
        setTimeout(() => {
          console.log("Pizza delivered! 🎉");
          resolve("Pizza!");
        }, 1000);
      });
    }
  
    function eatPizza(item) {
      return new Promise(resolve => {
        setTimeout(() => {
          console.log(`Mmm, eating ${item}... 🤤`);
          resolve("Empty plate!");
        }, 800);
      });
    }
  
    function cleanUp(result) {
      return new Promise(resolve => {
        setTimeout(() => {
          console.log(`Time to clean up the ${result} 🧹`);
          resolve("All clean!");
        }, 500);
      });
    }
  
    // Promise Chaining:
    deliverPizza()
      .then(item => eatPizza(item)) // When pizza delivered, THEN eat it
      .then(result => cleanUp(result)) // When done eating, THEN clean up
      .then(finalStatus => console.log(`Task complete: ${finalStatus} ✅`))
      .catch(error => console.error("Something went wrong in the process:", error)); // Catch any error along the chain
  

  This is much cleaner than deeply nested callbacks! ✨

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8. async/await: Async Code That Looks Like Sync Code! ⏸️🪄

async and await are special keywords introduced in JavaScript to make working with Promises even easier and make asynchronous code look and behave almost like synchronous code, making it much more readable.

• Interview Definition:

  • The async keyword is used to define an asynchronous function. An async function always returns a Promise.

  • The await keyword can only be used inside an async function. It pauses the execution of the async function until the Promise it's waiting for settles (either resolves or rejects).

• Analogy: The Magical Pause Button! ⏸️✨ Imagine you're watching a movie (your code running).

  • async is like getting a magic remote control for the movie. Only with this remote can you use the special await button.

  • await is the magic "pause" button on that remote. When you press await before a task that takes time (a Promise), your movie (code) pauses just for a moment, waits for that specific task to finish, and then automatically resumes right where it left off, without freezing the entire TV (the rest of your program)!

• Where to Use async and await:

  • Put async before the function keyword (or arrow function () =>).

  • Put await directly before any Promise-returning function call that you want to pause for.

• Example (Converting Promise Chaining to async/await): Let's use our pizza delivery example from before:

    // (Assume deliverPizza, eatPizza, cleanUp functions from previous Promise example exist)
  
    async function handlePizzaDay() { // 👈 Mark the function as 'async'
      try { // Use try...catch for error handling with async/await
        console.log("Starting the pizza day adventure! 🍕");
        const pizzaItem = await deliverPizza(); // 👈 'await' pauses here until pizza is delivered
        console.log("Got the pizza, now to eat!");
  
        const eatResult = await eatPizza(pizzaItem); // 👈 'await' pauses until pizza is eaten
        console.log("Done eating, now to clean!");
  
        const cleanResult = await cleanUp(eatResult); // 👈 'await' pauses until clean up is done
        console.log(`Pizza day complete: ${cleanResult} ✅`);
      } catch (error) {
        console.error("Oh no, something went wrong with pizza day:", error);
      }
    }
  
    handlePizzaDay(); // Call our async function to start the adventure!
    console.log("I'm doing other things while the pizza day unfolds in the background!");
  

  💬 Output:

    I'm doing other things while the pizza day unfolds in the background!
    Starting the pizza day adventure! 🍕
    Pizza delivered! 🎉 (after 1 second)
    Got the pizza, now to eat!
    Mmm, eating Pizza!... 🤤 (after another 0.8 seconds)
    Done eating, now to clean!
    Time to clean up the Empty plate! 🧹 (after another 0.5 seconds)
    Pizza day complete: All clean! ✅
  

  See how handlePizzaDay looks like synchronous code, making it much easier to read the flow, even though it's still doing asynchronous things in the background! This is the power of async/await! 🤩

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9. Advanced Concepts: Multitasking & Control! 🏃‍♀️💨

Beyond the core async mechanisms, there are some cool concepts for managing how and when your code runs, especially in performance-sensitive situations.

9.1 Concurrency vs. Parallelism: Juggling vs. Multiple Jugglers! 🤹‍♀️🤹‍♂️

These terms are often confused!

• Concurrency:

  • Definition: Means handling multiple tasks at once by switching between them very quickly (interleaving their execution). It's like a single chef multitasking – chopping veggies, stirring sauce, then checking the oven, all by rapidly switching attention. They're not doing everything at the exact same second, but they're managing multiple things simultaneously.

  • JavaScript: JavaScript (being single-threaded) achieves concurrency through the Event Loop! It manages multiple async operations by switching to them when they're ready, while its single main thread still does one thing at a time. 🔄

• Parallelism:

  • Definition: Means literally doing multiple tasks at the exact same time, usually on different computer cores or processors. It's like having multiple chefs working in the kitchen at once, each cooking a different dish simultaneously.

  • JavaScript: Pure JavaScript itself doesn't offer parallelism directly (because it's single-threaded). However, modern web browsers and Node.js environments can leverage parallelism through things like Web Workers, which run JavaScript in a separate thread. But the main JavaScript engine typically handles concurrency. 🧑‍🍳🧑‍🍳

9.2 Throttling: The "Bouncer" for Your Code! 🚶‍♂️🚪

• Interview Definition: Throttling is a technique used to limit how many times a function can be called within a given time period. It ensures a function executes at most once per a specified interval.

• Analogy: The Club Bouncer! 🕺💃 Imagine there's a popular club, and a bouncer at the door. The bouncer only lets one person in every 5 seconds. Even if 100 people rush the door, only one gets in every 5 seconds. That's throttling!

• Why Use It? To prevent a function from firing too rapidly, which can be bad for performance (e.g., resizing a window, scrolling events).

• Example (Conceptual):

    function handleScroll() {
      console.log("Scrolling the page!");
      // This function might run hundreds of times per second without throttling!
    }
  
    // Imagine a 'throttle' helper function:
    // const throttledScroll = throttle(handleScroll, 200); // Allow handleScroll at most once every 200ms
    // window.addEventListener('scroll', throttledScroll);
  

  (Implementing a throttle function involves a bit more code, usually a timer and a flag, but the concept is key!)

9.3 Debouncing: The "Snooze Button" for Your Code! ⏰😴

• Interview Definition: Debouncing is a technique used to delay the execution of a function until a certain amount of time has passed since the last time it was invoked. It ensures a function is called only after a period of inactivity.

• Analogy: The Snooze Button! 😴🔔 You hit the snooze button on your alarm clock. The alarm doesn't ring right away; it waits. If you hit snooze again, it resets the waiting time. The alarm only rings when you stop hitting snooze for a certain period. That's debouncing!

• Why Use It? Perfect for events that fire very rapidly and you only care about the final state after the user has stopped performing the action (e.g., typing in a search bar, resizing a window).

• Example (Conceptual):

    function performSearch(query) {
      console.log(`Searching for: ${query}...`);
      // This is the expensive function that hits a server
    }
  
    // Imagine a 'debounce' helper function:
    // const debouncedSearch = debounce(performSearch, 300); // Wait 300ms after last key press
    // document.getElementById('search-input').addEventListener('keyup', (e) => {
    //   debouncedSearch(e.target.value);
    // });
  

  (Like throttling, implementing debounce requires a timer that gets cleared and reset.)

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Phew! That was an epic journey into Asynchronous JavaScript! You've just learned how to make your programs super efficient, responsive, and how to handle the flow of time in your code like a true wizard. Keep practicing, and these concepts will feel like second nature! 💪🌟

Summary

Here's your action-packed summary for Day 6! 🚀✨

Today, we learned:

• Sync vs. Async Code: The big difference between doing tasks one after another (synchronous 🚶‍♀️) and juggling multiple tasks at once without waiting (asynchronous 🤹‍♀️).

• Why Async JS is Essential: How it keeps our webpages smooth and responsive when dealing with long tasks like fetching data from the internet 🌐 or setting timers ⏱️.

• Callbacks: Our first way to handle async results, like leaving a sticky note for instructions ("Call me back!"). 📝📞 We even peeked at "Callback Hell"! 🔥

• JavaScript's Secret: The Event Loop! 🤫 We uncovered how single-threaded JavaScript performs "multitasking" with the help of the browser/Node.js environment and the super important Event Loop manager! 🧑‍🍳🔄

• Promises: The modern, cleaner way to manage asynchronous operations, treating them like a "future result" object that can be pending, fulfilled, or rejected! 🤝🍕

• async/await: The magical keywords that make asynchronous code look and feel like synchronous code, making it incredibly easy to read and manage with try...catch blocks! ⏸️✨

• Concurrency vs. Parallelism: We understood the difference between juggling many tasks (concurrency 🏃‍♀️💨) and truly doing them at the exact same time (parallelism 🧑‍🍳🧑‍🍳).

• Throttling & Debouncing: Cool techniques to control how often our functions run, like a bouncer at a club (throttling 🚶‍♂️🚪) or a snooze button on an alarm (debouncing ⏰😴)!

What a day of deep dives into JavaScript's superpower world! You're becoming a true async wizard! 💪🧠

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Follow more at: Ravindranath Porandla Blog 🧠💖

— Ravindranath Porandla 🧑‍💻