Q275- What is an IIFE (Immediately Invoked Function Expression)? Why and when is it used?

An IIFE is a JavaScript function that is defined and executed immediately after it is created.

Syntax:

(function () {
  console.log("IIFE executed");
})();

or (arrow function):

(() => {
  console.log("IIFE executed");
})();

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✅ Why Is IIFE Used?

✔ 1. Avoid Global Scope Pollution

Variables inside an IIFE stay private.

(function () {
  var secret = "hidden";
})();
console.log(secret); // ❌ ReferenceError

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✔ 2. Create a Private Scope

Before let and const, IIFE was the main way to create block-like scope.

(function () {
  var count = 0;
  console.log(count);
})();

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✔ 3. Execute Code Only Once

Useful for:

• initialization logic
• configuration setup
• one-time calculations

const config = (() => {
  return { apiUrl: "https://api.com" };
})();

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✔ 4. Avoid Variable Name Conflicts

Especially useful in large codebases or older JS.

(function () {
  var name = "Teekam";
})();

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✔ 5. Used in Module Patterns

Before ES6 modules.

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

  return {
    increment() {
      count++;
      return count;
    }
  };
})();

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🔥 When Should You Use IIFE?

✔ One-time setup code
✔ Protect variables from global scope
✔ Creating private data
✔ Legacy JavaScript (pre-ES6)
✔ Library initialization

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❌ When NOT Needed Today?

• When using ES6 modules (import/export)
• When using block scope (let, const)

Modern code relies less on IIFE, but it’s still important to understand.

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🎯 Short Interview Answer

An IIFE is a function that runs immediately after being defined.
It is used to create a private scope, avoid polluting the global namespace, and run one-time initialization logic.
IIFEs were widely used before ES6 modules and let/const.