⚙️ Day 7: Introduction to Asynchronous Programming in Node.js (Using Callbacks)

👨‍🏫 What You’ll Learn Today

• What is synchronous vs asynchronous programming

• What are callbacks and why we use them

• How Node.js handles async code

• Real-life practical examples

• Common mistakes and how to fix them

• FAQs to clear all confusion

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🧠 Why Asynchronous Programming?

In Node.js, most tasks like:

• reading a file

• sending an email

• making a database call

...take time ⏳

If we wait for each task to complete before starting the next one, the app will become slow and unresponsive.
That’s where asynchronous programming helps!

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🕒 Synchronous vs Asynchronous

🔴 Synchronous (Blocking)

console.log("1");
console.log("2");
console.log("3");

✅ Output:

1
2
3

Everything runs one after another, blocking the next line.

---

🟢 Asynchronous (Non-blocking)

console.log("1");

setTimeout(() => {
  console.log("2 (after delay)");
}, 1000);

console.log("3");

✅ Output:

1
3
2 (after delay)

🧠 Here, setTimeout is asynchronous — it lets the rest of the code run without waiting.

---

🔄 What is a Callback?

A callback is a function passed as an argument to another function, to be called later when the task is done.

👉 Think of it like this:

"Hey Node.js, go read this file. When you're done, call this function."

---

📁 Real-Life Example: Reading a File (Async vs Sync)

🟡 Sync (blocking)

const fs = require('fs');

const data = fs.readFileSync('info.txt', 'utf-8');
console.log(data); // File content
console.log("This runs after file read");

Everything waits until file is fully read. Not good for large files or high traffic.

---

🟢 Async with Callback (non-blocking)

const fs = require('fs');

fs.readFile('info.txt', 'utf-8', (err, data) => {
  if (err) return console.error(err);
  console.log(data); // File content
});

console.log("This runs immediately!");

✅ Output:

This runs immediately!
(File content appears later)

---

🔧 Callback Structure

function task(callback) {
  // Do something
  callback(); // Call the function passed in
}

Example:

function greet(name, callback) {
  console.log(`Hi, ${name}`);
  callback();
}

function bye() {
  console.log("Goodbye!");
}

greet("Payal", bye);

✅ Output:

Hi, Payal
Goodbye!

---

🧠 Real-World Use: Simulating a User Registration System

function registerUser(name, callback) {
  setTimeout(() => {
    console.log(`✅ User ${name} registered!`);
    callback(); // call next step
  }, 2000);
}

function sendWelcomeEmail() {
  console.log("📧 Welcome email sent!");
}

registerUser("Payal", sendWelcomeEmail);

✅ Output:

✅ User Payal registered!
📧 Welcome email sent!

---

🚫 Common Callback Mistake: Callback Hell

When we nest too many callbacks, it becomes hard to read:

step1(() => {
  step2(() => {
    step3(() => {
      step4(() => {
        // 😩 deep and messy!
      });
    });
  });
});

💡 This is called callback hell. We’ll solve it using Promises and Async/Await in upcoming lessons.

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✅ Where Are Callbacks Commonly Used?

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❓ FAQs – Frequently Asked Questions

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1) Why not always use synchronous (Sync) versions?

Synchronous methods block your server. For example:

• One user uploading a big file can freeze the server for others.

• Async with callbacks keeps everything fast and smooth.

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2) What is the difference between callback() and callback?

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3) Is setTimeout() async or sync?

✅ It’s asynchronous. It lets the program continue without waiting for the timer.

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4) Can we return a value from a callback?

No, you can't return a value out of an async callback like normal functions.
Instead, you use the data inside the callback.

❌ Doesn’t work:

let data = fs.readFile('file.txt', 'utf-8', (err, result) => {
  return result;
});
console.log(data); // undefined!

✅ Correct:

fs.readFile('file.txt', 'utf-8', (err, result) => {
  console.log(result); // use it here
});

---

📝 Practice Tasks

1. Create a function fetchData that takes a callback and logs a message after 2 seconds.

2. Read any file using both readFileSync and readFile – compare the order.

3. Simulate user login, email verification, and welcome message using nested callbacks.

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✅ Summary

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🎉 Congratulations! You’ve completed your first step into asynchronous programming in Node.js.

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