Q320- What is IIFE in JavaScript? Why we need it in JavaScript?

IIFE (Immediately Invoked Function Expression) is a JavaScript function that is defined and executed immediately after it is created.

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🔹 What is an IIFE?

(function () {
  console.log("I run immediately");
})();

or (arrow function):

(() => {
  console.log("I run immediately");
})();

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🧠 Why We Need IIFE in JavaScript

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1️⃣ Avoid Polluting the Global Scope

Before ES6, JavaScript had no block scope.

var count = 10;

Using IIFE:

(function () {
  var count = 10;
})();

✔ count is private
✔ Not accessible globally

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2️⃣ Create Private Variables (Encapsulation)

const counter = (function () {
  let count = 0;

  return {
    increment() {
      count++;
      console.log(count);
    }
  };
})();

counter.increment(); // 1
counter.increment(); // 2

✔ Data privacy
✔ Closure-based state

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3️⃣ Execute Code Immediately

Useful for setup logic.

(function init() {
  console.log("App initialized");
})();

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4️⃣ Avoid Variable Name Conflicts

Especially in large or third-party scripts.

(function () {
  var user = "Admin";
})();

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5️⃣ Module Pattern (Before ES Modules)

IIFE was used to simulate modules.

const app = (function () {
  function start() {
    console.log("Started");
  }
  return { start };
})();

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❓ Why Parentheses Are Needed

JavaScript treats function() as a declaration.
Wrapping it in () turns it into an expression, which can be executed immediately.

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❌ Do We Still Need IIFE Today?

Less than before, because:

• let and const provide block scope
• ES Modules provide proper modules

But IIFE is still useful: ✔ For quick isolated execution
✔ In legacy code
✔ For one-time setup logic

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🎯 Short Interview Answer

An IIFE is a function that runs immediately after it is defined.
It is used to create a private scope, avoid global variable pollution, and execute setup code instantly.
Before ES6 modules, IIFEs were commonly used to implement the module pattern.

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⭐ One-line summary

IIFE creates an isolated scope and runs immediately to protect variables and avoid global pollution.