Recursive Functions

Recursion occurs when a function invokes itself to solve a smaller form of the same problem. Every recursive path needs a reachable base case.

What you will learn: the purpose of recursive functions, the rules TypeScript checks, and what its JavaScript behavior looks like in a runnable example.

The factorial example computes n × factorial(n−1) until n is 1. Each invocation gets a separate parameter binding and execution frame. Large recursion depths may exhaust the JavaScript call stack; loops are often more appropriate for unbounded iteration.

Key distinction: TypeScript checks the declared types during development; emitted JavaScript carries out the value-level operation at runtime. A type declaration alone never validates untrusted runtime input.

Read this illustration from the first labeled part to the last. Compare it with the example and identify which parts are type checks and which parts run in JavaScript.

This self-contained example uses fixed values, so it runs without reading input or depending on an external service. Paste it into the TypeScript Playground or save it as main.ts.

TypeScript program

function factorial(n: number): number {
  if (n <= 1) return 1;
  return n * factorial(n - 1);
}
console.log(factorial(4));

Verified output

24

Run locally: npx tsc --strict --target es2022 --module commonjs main.ts, then node main.js. For projects with a tsconfig.json, build using that configuration instead.

factorial(4) waits for 4 × factorial(3), which waits for 3 × factorial(2), then 2 × factorial(1). The base call returns 1; multiplying on the way back gives 24.

Check each step: identify the input values, follow the JavaScript operation in execution order, and match the final expression with the exact output shown. An annotation can reject invalid source before execution, but does not change the value printed.

Do not omit or make the base case unreachable; infinite recursion eventually raises a runtime RangeError.

How to debug: reproduce the issue in a minimal example. For compiler errors, inspect the inferred and declared types; for a runtime surprise, log the actual values and follow their references or branches.

Draw the call tree for factorial(5). Then write a recursive sumUpTo(n) and compare it with a loop.

Practice routine: predict the result on paper, edit one part of the code, compile under --strict, run the JavaScript, and explain whether TypeScript caught an error or JavaScript simply behaved differently.

Recursive Functions: Recursion occurs when a function invokes itself to solve a smaller form of the same problem.

factorial(4) waits for 4 × factorial(3), which waits for 3 × factorial(2), then 2 × factorial(1). The base call returns 1; multiplying on the way back gives 24.

Before continuing, explain one valid use case, one common pitfall, and the separation between compile-time checking and runtime behavior for this feature.