What is recursion and how is it used in JavaScript?

Recursion is a programming technique where a function calls itself to solve a problem. In JavaScript, recursion is used to solve problems that can be broken down into smaller, similar sub-problems. A base case is essential to stop the recursive calls and prevent infinite loops. For example, calculating the factorial of a number can be done using recursion:

function factorial(n) {
  if (n === 0) {
    return 1;
  }
  return n * factorial(n - 1);
}

console.log(factorial(4)); // Output: 24