Express JS Tutorial – Validation

When building a web application with Express JS, it is important to validate user input to ensure data integrity and security. In this tutorial, we will explore how to perform validation in Express JS.

1. Using middleware for validation

Express JS provides middleware that can be used for request validation. One popular middleware for validation is express-validator. This middleware provides a set of validation and sanitization functions that can be used to validate and sanitize user input.

First, install the express-validator package using npm:

npm install express-validator
    

Then, use the middleware in your Express application:

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

app.post('/user', 
    body('username').isEmail(),
    (req, res) => {
    // Handle the request
});
    

2. Custom validation functions

In addition to using the built-in validation functions provided by express-validator, you can also create custom validation functions to perform more specific validation logic.

For example, you can create a custom validation function to check if a user’s age is above a certain threshold:

const validateAge = (value) => {
    if (value < 18) {
        throw new Error('Age must be at least 18');
    }
};
    

Then, use the custom validation function in your route handler:

app.post('/user', 
    body('age').custom(validateAge),
    (req, res) => {
    // Handle the request
});
    

3. Handling validation errors

After performing validation, you can check for validation errors using the validationResult function provided by express-validator:

app.post('/user', 
    body('username').isEmail(),
    (req, res) => {
    const errors = validationResult(req);
    if (!errors.isEmpty()) {
        return res.status(400).json({ errors: errors.array() });
    }
    // Handle the request
});
    

By checking for validation errors and returning an appropriate response, you can ensure that only valid data is processed by your application.

In summary, validation is an important aspect of building secure and robust web applications with Express JS. By using middleware for validation, creating custom validation functions, and handling validation errors, you can ensure that user input is validated effectively.