is-jwt-token

A lightweight utility to validate JWT (JSON Web Token) format. This package provides a single function to quickly check if a string is a properly formatted JWT token.

Features

• ✅ Zero dependencies - Lightweight and secure
• ✅ Single function - Simple and focused API
• ✅ Fast validation - Optimized for performance
• ✅ TypeScript friendly - Works with TypeScript projects
• ✅ Comprehensive validation - Validates structure, encoding, and JSON format
• ✅ Production ready - Thoroughly tested

Installation

npm install is-jwt-token

Usage

const isJWT = require('is-jwt-token');

// Valid JWT token
const validToken = 'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c';

console.log(isJWT(validToken)); // true

// Invalid tokens
console.log(isJWT('invalid.token')); // false
console.log(isJWT('not-a-token')); // false
console.log(isJWT(null)); // false
console.log(isJWT('')); // false
console.log(isJWT('   ')); // false (whitespace only)

API

isJWT(token)

Validates if a string is a properly formatted JWT token.

Parameters:

• token (any): The token to validate

Returns: boolean - True if token is valid JWT format, false otherwise

Examples

Basic Usage

const isJWT = require('is-jwt-token');

const userToken = 'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIn0.abc123';

if (isJWT(userToken)) {
  console.log('Valid JWT token format');
  // Proceed with token verification using other libraries
} else {
  console.log('Invalid token format');
}

Express.js Middleware

const isJWT = require('is-jwt-token');

function validateJWTFormat(req, res, next) {
  const authHeader = req.headers.authorization;
  const token = authHeader?.startsWith('Bearer ') 
    ? authHeader.substring(7) 
    : null;
  
  if (!isJWT(token)) {
    return res.status(400).json({ 
      error: 'Invalid JWT token format' 
    });
  }
  
  req.token = token;
  next();
}

app.use('/api/protected', validateJWTFormat);

Input Validation

const isJWT = require('is-jwt-token');

function processToken(userInput) {
  if (!isJWT(userInput)) {
    throw new Error('Invalid JWT format');
  }
  
  // Continue with signature verification, etc.
  return verifyTokenSignature(userInput);
}

What This Package Validates

✅ Token structure - Ensures exactly 3 parts separated by dots
✅ Base64url encoding - Validates proper encoding of each part
✅ JSON format - Ensures header and payload are valid JSON
✅ Required fields - Checks header contains 'alg' property
✅ Edge cases - Handles null, undefined, empty strings, whitespace

What This Package Does NOT Do

❌ Signature verification - Does not verify cryptographic signatures
❌ Token generation - Does not create JWT tokens
❌ Expiration checking - Does not validate exp claims
❌ Claim validation - Does not validate token claims

Note: This package only validates JWT format. For complete JWT functionality including signature verification, use libraries like jsonwebtoken.

License

MIT