Recursion Explained — Stop Fearing It in 10 Minutes

Recursion = a function calling itself, each time with a smaller problem, until the problem is trivial. Two parts, always:

The anatomy

function factorial(n) {
  if (n <= 1) return 1;          // BASE CASE — when to stop
  return n * factorial(n - 1);   // RECURSIVE CASE — shrink the problem
}
// factorial(4) → 4 × 3 × 2 × 1 = 24

Forget the base case → infinite calls → "Maximum call stack size exceeded". That error IS recursion's failure mode.

Trace it (the skill interviews test)

factorial(3)
  = 3 * factorial(2)
        = 2 * factorial(1)
              = 1              ← base case hit
        = 2 * 1 = 2
  = 3 * 2 = 6                  ← stack unwinds

When recursion shines vs hurts

• Shines: trees, nested structures (folders, JSON, DOM), divide-and-conquer (merge sort)
• Hurts: plain counting — naive fib(40) makes ~1.6 billion calls. Iterate or memoize instead

Practice: Fibonacci on our judge deliberately times out naive recursion at n=40 — beat it with iteration.