How can I create custom validation rules using express-validator?

How can I create custom validation rules using express-validator?

You can create custom validation rules in express-validator by using the check function along with a custom validator. Here's a basic example:

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

app.post('/user', [
  check('username').custom(value => {
    if (value === 'admin') {
      throw new Error('Username cannot be admin');
    }
    return true;
  })
], (req, res) => {
  const errors = validationResult(req);
  if (!errors.isEmpty()) {
    return res.status(400).json({ errors: errors.array() });
  }
  // Proceed with user creation
});

Can you explain how the custom validator works in that example?

Sure! In the example, the custom method allows you to define your own validation logic. The function receives the value of the field being validated. If the value meets certain conditions (like being 'admin'), you can throw an error. If it passes, return true.

What if I want to validate multiple fields at once?

You can use the check function multiple times for different fields or use body to validate multiple fields together. Here's an example:

app.post('/user', [
  check('username').notEmpty().withMessage('Username is required'),
  check('password').isLength({ min: 6 }).withMessage('Password must be at least 6 characters long')
], (req, res) => {
  const errors = validationResult(req);
  if (!errors.isEmpty()) {
    return res.status(400).json({ errors: errors.array() });
  }
  // Proceed with user creation
});