What the Heck is an IIFE in JavaScript?

If you've been around JavaScript long enough, you’ve probably seen something like this:

(function () {
  console.log("I'm an IIFE!");
})();

It looks weird at first, but it’s doing something cool. Let’s break it down.

What’s an IIFE?

IIFE stands for Immediately Invoked Function Expression. It’s a function that runs as soon as it’s defined.

Why? To create a private scope and avoid polluting the global namespace—especially useful before let and const were common.

How Does It Work?

There are two main parts:

1. Function Expression

Wrapping the function in parentheses tells JavaScript: “this is an expression.”

(function () { ... })

2. Immediate Invocation

These final parentheses call the function right away.

();

Put it together:

(function () {
  const message = "This runs immediately!";
  console.log(message);
})();

Should You Still Use Them?

Modern JavaScript gives us block scoping via let, const, and modules, so IIFEs aren’t as necessary as they used to be. But they’re still a great trick to have in your toolbox—especially in scripts or legacy codebases.