What is an IIFE (Immediately Invoked Function Expression) in JavaScript? What are the pros and cons?

What is an IIFE?

An IIFE (Immediately Invoked Function Expression) is a function that is defined and executed immediately.

// Basic syntax
(function() {
  // function body
})();

// Arrow function version
(() => {
  // function body
})();

// With parameters
(function(name) {
  console.log('Hello, ' + name);
})('World');

Why the Wrapping Parentheses?

The function must be wrapped in parentheses so the JavaScript parser treats it as an expression rather than a declaration. Without them, a leading function keyword is parsed as a function declaration, which cannot be immediately invoked.

Advantages

1. Avoids Polluting the Global Scope

(function() {
  var secret = '123'; // only exists inside the IIFE
})();

console.log(typeof secret); // 'undefined'

2. Creates a Private Scope

Simulates module encapsulation for variables and logic.

3. Prevents Variable Conflicts

Especially useful in legacy environments where multiple scripts coexist.

4. Runs Initialization Logic

Great for one-time setup code that runs immediately.

Disadvantages

1. Reduced readability: The syntax is obscure and not intuitive
2. Hard to test: Logic inside an IIFE is difficult to unit test
3. Modern alternatives exist: ES6 let/const have block scope, and ES modules (ESM) provide better encapsulation, greatly reducing the need for IIFEs

Modern Alternatives

// ES Modules (recommended)
// Variables in a module are private by default

// Block scope
{
  let secret = '123';
}
console.log(typeof secret); // 'undefined'