Which one to choose?

In summary, when it comes to validation in Express applications, different approaches are utilized based on specific scenarios. While Joi is commonly used, there are other options available as well.

Custom validation provides developers with complete control and flexibility over the validation process. It is suitable for cases where unique validation needs or fine-grained control are required. Custom validation allows developers to implement complex validation rules, integrate with external systems or services, and customize error messages and response formats.

Mongoose's validation, on the other hand, is particularly useful for developers using MongoDB as their database and Mongoose as the ODM. It allows for the definition of validation rules directly within the schema, ensuring data integrity at the database level. Mongoose's validation is beneficial for enforcing specific field-level validation rules and leveraging built-in error handling and validation messages.

While Joi is a popular choice, it is essential to evaluate the requirements of your application. If you require unique validation logic or want more control, custom validation is a suitable option. If you are using MongoDB with Mongoose, leveraging Mongoose's validation is recommended. However, for a powerful, standalone validation library with flexibility and reusability, Joi remains the preferred choice among developers for Express applications.