Recursion Programs

Recursion solves a problem by calling the same function on smaller inputs until a base case is reached. Each call consumes stack space, so recursion is not always the best solution for very deep inputs. Validate the input and guarantee the step moves toward the base case.

For 5, the calls calculate 5 × 4 × 3 × 2 × 1, returning 120. The 0! base case returns 1. An upper bound limits number precision problems.

Missing a base case or passing an input that does not approach it can cause unbounded recursion and a stack overflow.

This source was compiled using TypeScript 5.8.3 with strict checking, then executed with Node.js 22.16.0. Its stdout below is the actual captured output. The core logic is runnable without a framework or a remote API; integration-specific examples appear in a separate lab when needed.

TypeScript source

function factorial(n: number): number {
  if (!Number.isSafeInteger(n) || n < 0 || n > 20) throw new Error("n must be 0..20");
  if (n <= 1) return 1;
  return n * factorial(n - 1);
}
console.log(factorial(0));
console.log(factorial(5));

Verified output

1
120

Compile: npx tsc --strict --target es2022 --module commonjs recursion-programs.ts; run: node recursion-programs.js. For examples importing node: modules, install @types/node as a development dependency; for browser or React projects use the separate lab setup.

For 5, the calls calculate 5 × 4 × 3 × 2 × 1, returning 120. The 0! base case returns 1. An upper bound limits number precision problems.

Before editing the code, predict what the output will be for one changed input. Run it and compare your prediction with the actual result.

Missing a base case or passing an input that does not approach it can cause unbounded recursion and a stack overflow.

Determine whether the issue belongs to the type system, a trust boundary, or actual runtime behavior before choosing a correction.

Implement the greatest common divisor recursively using Euclid's algorithm, then compare against a loop.

Record expected success and failure outcomes, run the code, and compare the observed behavior. Keep the exercise compatible with strict type checking.

For 5, the calls calculate 5 × 4 × 3 × 2 × 1, returning 120. The 0! base case returns 1. An upper bound limits number precision problems.

Apply the same reasoning to the practice task, then integrate it into a real app only when the runtime and dependency setup are ready.