JWT Tokens Explained: A Developer's Complete Guide

What Are JWT Tokens? JSON Web Tokens (JWT) are a compact, URL-safe way to represent claims between two parties. They are widely used for authentication and authorization in modern web applications. Instead of storing session data on the server, JWTs allow stateless authentication by encoding user information directly into the token. JWT Structure: Three Parts A JWT consists of three Base64URL-encoded parts separated by dots: header.payload.signature 1. Header The header specifies the token type and the signing algorithm: { "alg" : "HS256" , "typ" : "JWT" } 2. Payload The payload contains the claims — statements about the user and additional metadata: { "sub" : "1234567890" , "name" : "Alice Developer" , "iat" : 1516239022 , "exp" : 1516242622 , "role" : "admin" } Common registered claims include: iss — Issuer sub — Subject (user ID) exp — Expiration time iat — Issued at aud — Audience 3. Signature The signature ensures the token has not been tampered with: HMACSHA256( base64UrlEncode(h