A function in JavaScript is a block of code designed to perform a specific task. You can reuse a function wherever you need it, which helps keep your code clean and efficient.

Function Structure:

1. Function Declaration (named function)

 2. Function with Parameters

3. Function with Return Value

4. Function Expression (anonymous function)

 5. Arrow Function (ES6)

A function can be called any number of times, and it only runs when called.