What methods do you use to verify data correctness in an Express.js application?

To verify data correctness in an Express.js application, the following methods are usually used:

• Route-level validation with middleware. For example, libraries like express-validator allow defining rules for request fields (body, params, query).

• Schema validation with libraries like Joi or Yup, which describe data structure and constraints, validating before processing.

• Type and format checks (e.g., email, numbers, dates) using regex or built-in functions.

• Validation error handling — if data fails validation, return a clear response with error details (e.g., 400 Bad Request).

Example with express-validator:

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

app.post('/user', [
  body('email').isEmail(),
  body('age').isInt({ min: 0 })
], (req, res) => {
  const errors = validationResult(req);
  if (!errors.isEmpty()) {
    return res.status(400).json({ errors: errors.array() });
  }
  // Process valid data
  res.send('Data is valid');
});

Thus, data validation in Express.js relies on middleware and specialized libraries to ensure input correctness and security.