What is an IIFE (Immediately Invoked Function Expression) and when do you use it?

An IIFE is a function that is both defined and invoked immediately. It creates an isolated scope.

// Classic IIFE syntax
(function() {
  const private = 'inaccessible outside';
  console.log(private);
})();

// Arrow IIFE
(() => {
  // isolated scope
})();

// IIFE with parameters
(function(global) {
  global.myLib = {};
})(window);

// IIFE returning a value
const result = (() => {
  const x = computeExpensiveThing();
  return x * 2;
})();

Use cases:

• Avoid polluting global scope (classic library pattern)
• Create truly private variables (module pattern)
• Capture loop variables (pre-let closure fix)
• One-time initialization logic