How do you validate data in Node.js applications?

Data validation can be done using libraries like Joi, express-validator, or validator.js. These libraries help ensure that the data received is in the correct format before processing. Example using express-validator:

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

app.post('/user', [
  body('email').isEmail(),
  body('password').isLength({ min: 5 })
], (req, res) => {
  const errors = validationResult(req);
  if (!errors.isEmpty()) {
    return res.status(400).json({ errors: errors.array() });
  }
  res.send('User is valid');
});