JWT, or JSON Web Token, has become the go‑to method for securing modern web apps. The token is a compact, URL‑safe string split into header, payload, and signature. The header declares the algorithm, the payload carries claims like user ID, and the signature verifies integrity.

In practice, a user submits email and password to a Node.js server running Express. After verifying credentials, the server signs a JWT with auth-verify, sets an expiresIn of one hour, and returns the token. The client stores it in memory or a cookie and sends it in the Authorization header on subsequent requests.

Developers often fall into pitfalls: storing tokens in localStorage exposes them to XSS, embedding sensitive data in the payload, neglecting expiration, or using weak secrets. Best practice recommends HTTP‑only cookies, rotating secrets in production, and always setting expiresIn to limit token lifespan.

JWT shines when building stateless APIs, microservices, or SPAs that need mobile authentication. It falls short if instant global logout or heavy session control is required. Mastering JWT equips developers to create secure, scalable backends that can evolve with growing application demands.