What is IIFE (Immediately Invoked Function Expression) in JavaScript

IIFE (Immediately Invoked Function Expression) is an immediately invoked function expression in JavaScript. It's a function that is defined and immediately called as soon as the interpreter reaches it.

Syntax

(function () {
  // code inside IIFE
})();

Or using arrow function:

(() => {
  // code inside IIFE
})();

Why Use IIFE?

• Isolates variables and functions from outer scope.
• Used to create local scope, especially before let and const appeared.
• Often used in modular development when you need to hide internal implementation details.

Usage Example

(function () {
  const message = "Hello from IIFE!";
  console.log(message); // "Hello from IIFE!"
})();

console.log(message); // ReferenceError: message is not defined

Variable message is only available inside IIFE and inaccessible outside — this avoids polluting global scope.

IIFE with Arguments

(function (name) {
  console.log(`Hello, ${name}!`);
})("Alice"); // Hello, Alice!

When is it Used?

• For one-time data initialization (e.g., setup)
• For creating private variables and closures
• In old pre-ES6 projects for module simulation

Example: counter with closure via IIFE

const counter = (function () {
  let count = 0;
  return function () {
    count++;
    console.log(count);
  };
})();

counter(); // 1
counter(); // 2

Summary

IIFE is a function that is immediately called after definition.

• Used for code isolation and creating private namespace.
• It's a useful technique for writing modular and safe code.