How can I validate user input in a TypeScript Express application?

How can I validate user input in a TypeScript Express application?

You can use libraries like express-validator or joi to validate user input in your TypeScript Express app.

Can you show me an example using express-validator?

Sure! First, install it using npm: npm install express-validator. Then, you can use it like this:

import { body, validationResult } from '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() });
  }
  // Proceed with user creation
});

What if I want to create a custom validation rule?

You can create a custom validator by using check and defining your own validation logic. For example:

import { check } from 'express-validator';

app.post('/user', [
  check('username').custom(value => {
    if (value === 'admin') {
      throw new Error('Username cannot be admin');
    }
    return true;
  })
], (req, res) => {
  // Handle request
});