How to Validate Request Body with express-validator

Problem

You need to validate incoming JSON data in POST requests to ensure required fields exist, have correct types, and meet specific criteria before processing.

Solution

const express = require('express');
const { body, validationResult } = require('express-validator');
const app = express();

app.use(express.json());

// Validation middleware
const validateUser = [
  body('email')
    .isEmail()
    .withMessage('Must be a valid email')
    .normalizeEmail(),
  body('password')
    .isLength({ min: 6 })
    .withMessage('Password must be at least 6 characters'),
  body('age')
    .isInt({ min: 18, max: 100 })
    .withMessage('Age must be between 18 and 100')
];

// Route with validation
app.post('/api/users', validateUser, (req, res) => {
  // Check for validation errors
  const errors = validationResult(req);
  if (!errors.isEmpty()) {
    return res.status(400).json({
      success: false,
      errors: errors.array()
    });
  }

  // Process valid data
  const { email, password, age } = req.body;
  res.json({
    success: true,
    message: 'User created successfully',
    user: { email, age }
  });
});

app.listen(3000, () => {
  console.log('Server running on port 3000');
});

Install express-validator:

npm install express-validator

Explanation

express-validator provides middleware functions that validate request data. Each body() call validates a specific field with built-in validators like isEmail(), isLength(), and isInt().

validationResult(req) collects all validation errors. If errors exist, return a 400 status with error details. The validation runs before your route handler, so you always receive clean, validated data when validation passes.