URL Shortener

A URL shortener maps a short code to a validated destination URL. A complete service needs uniqueness, persistence, abuse prevention, and safe redirect handling. This example creates predictable unique short codes in memory and demonstrates lookup; production codes should be designed for collision behavior and abuse resistance.

The URL constructor checks syntax, the protocol guard allows only HTTP(S), and the incrementing counter produces the first short code 1. A real HTTP redirect uses an appropriate 3xx response.

Never redirect to arbitrary javascript: URLs, and do not trust sequential in-memory IDs for a real multi-instance public service.

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

class UrlShortener {
  private readonly links = new Map<string, string>();
  private next = 1;
  create(rawUrl: string): string {
    const url = new URL(rawUrl);
    if (url.protocol !== "https:" && url.protocol !== "http:") throw new Error("Unsupported protocol");
    const code = (this.next++).toString(36);
    this.links.set(code, url.toString());
    return code;
  }
  resolve(code: string): string | undefined { return this.links.get(code); }
}
const shortener = new UrlShortener();
const code = shortener.create("https://example.com/docs");
console.log(code);
console.log(shortener.resolve(code));

Verified output

1
https://example.com/docs

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

The URL constructor checks syntax, the protocol guard allows only HTTP(S), and the incrementing counter produces the first short code 1. A real HTTP redirect uses an appropriate 3xx response.

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

Never redirect to arbitrary javascript: URLs, and do not trust sequential in-memory IDs for a real multi-instance public service.

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

Add custom aliases with duplicate detection and persistence. Return 404 when resolving an unknown code.

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

The URL constructor checks syntax, the protocol guard allows only HTTP(S), and the incrementing counter produces the first short code 1. A real HTTP redirect uses an appropriate 3xx response.

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