Full Stack Development, a quite tough to explain in a couple of lines. It takes ownership of the entire application from top to bottom. If you see a person as Full Stack Developer, he must be an all-rounder to develop an application from zero to high excellence. Full Stack Development includes 2 layers of architecture :

  1. Front-End Development (aka Client-Side / Presentation Layer)
  2. Back-End Development (aka Server-Side / Business Logic Layer)

Each Layer should have a data validations. Let’s dive in deeper,

Client-Side Validations

Validation takes place on the client side(web browser). It would be more frustrating to wait for the server response and re-enter the form that has nearly 100 inputs. So it is mainly for formatting data instantly and for the quick error recovery process.
 Client-side validation gives the user feedback ASAP without having to wait for the server. It is an added topping, but not necessary. This will avoid the majority of validation issues and unnecessary processing from the server to perform data validations.

Pros:

  • Faster than Server-Side Validation
  • Better user experience can be provided by responding quickly(like invalid phone number, required fields,etc..)
  • Saves network bandwidth, traffic API calls

Cons:

  • Not Secure and useless if client-side scripts are disabled(javascript disabled)

Server-Side Validations
It is all about the data validation takes place on the server-side using the server-side scripting language. If you use server-side validation, you are developing a more secure application. Because client-side validations can be bypassed easily.
Let’s take a scenario. If a user is a technical guy/ a malicious user, he can pass invalid inputs through API testing tool or he can disable the javascript in client script or some malicious attack to skip client side validations. In these cases, validation will not happen. This may lead to incorrect entries, server downtime if any serious errors, etc, So we should verify on the server as well.
There are few libraries(eg., express-validator, etc) available to validate the inputs. Better, write a few code snippets for validations before continuing your business logic like 

router.post(‘/api’, async(req, res) => {
 let isValidationPassed= await validateAndHandleInputs(req);
 if(isValidationPassed){
  //continue the business logics
 }
})

Pros:

  • More Secure than Client-Side Validation
  • Validation Techniques and Logics cannot be viewed/modified by the user.

Cons

  • Comparatively slower execution

References:

  1. https://www.smashingmagazine.com/2009/07/web-form-validation-best-practices-and-tutorials/
  2. https://medium.com/@davidpetri/server-and-client-side-validation-with-javascript-html-and-hapi-js-eccc779e448a
  3. http://net-informations.com/faq/asp/validation.htm
  4. https://surajdeshpande.wordpress.com/2013/08/10/difference-between-server-side-validation-and-client-side-validation/