Lesson 20 — Recursion

A function that calls itself

Recursion solves a problem by solving a smaller version of the same problem. It needs a base case — an input small enough to answer directly — or it never stops. Factorial: 5! = 5 × 4 × 3 × 2 × 1, and n! = n × (n-1)!, with 1! = 1 as the base case.

factorial(3) → 3 * factorial(2) → 3 * 2 * factorial(1) → 3 * 2 * 1

Why it matters: nested things — folders, menus, comment threads — are naturally recursive.