[removed] client-side vs. server-side validation

Question

Which is better to do client side or server side validation?

In our situation we are using

• jQuery and MVC.
• JSON data to pass between our Vi

Answer 1

You can do Server side validation and send back a JSON object with the validation results for each field, keeping client Javascript to a minimum (just displaying results) and still having a user friendly experience without having to repeat yourself on both client and server.

Answer 2

Client side should use a basic validation via HTML5 input types and pattern attributes and as these are only used for progressive enhancements for better user experience (Even if they are not supported on < IE9 and safari, but we don't rely on them). But the main validation should happen on the server side..

Answer 3

I am just going to repeat it, because it is quite important:

Always validate on the server

and add JavaScript for user-responsiveness.

Answer 4

I came across an interesting link that make a distinction between gross, systematic, random errors.

Client-Side validation suits perfectly for preventing gross and random errors. Typically a max length for texture and input. Do not mimic the server-side validation rule; provide your own gross, rule of thumb validation rule (ex. 200 characters on client-side; n on server-side dictated by a strong business rule).

Server-side validation suits perfectly for preventing systematic errors; it will enforce business rules.

In a project I'm involved in, the validation is done on the server through ajax requests. On the client I display error messages accordingly.

Further reading: gross, systematic, random errors:

https://answers.yahoo.com/question/index?qid=20080918203131AAEt6GO

Answer 5

If you are doing light validation, it is best to do it on the client. It will save the network traffic which will help your server perform better. If if it complicated validation that involves pulling data from a database or something, like passwords, then it best to do it on the server where the data can be securely checked.

Answer 6

As others have said, you should do both. Here's why:

Client Side

You want to validate input on the client side first because you can give better feedback to the average user. For example, if they enter an invalid email address and move to the next field, you can show an error message immediately. That way the user can correct every field before they submit the form.

If you only validate on the server, they have to submit the form, get an error message, and try to hunt down the problem.

(This pain can be eased by having the server re-render the form with the user's original input filled in, but client-side validation is still faster.)

Server Side

You want to validate on the server side because you can protect against the malicious user, who can easily bypass your JavaScript and submit dangerous input to the server.

It is very dangerous to trust your UI. Not only can they abuse your UI, but they may not be using your UI at all, or even a browser. What if the user manually edits the URL, or runs their own Javascript, or tweaks their HTTP requests with another tool? What if they send custom HTTP requests from curl or from a script, for example?

(This is not theoretical; eg, I worked on a travel search engine that re-submitted the user's search to many partner airlines, bus companies, etc, by sending POST requests as if the user had filled each company's search form, then gathered and sorted all the results. Those companies' form JS was never executed, and it was crucial for us that they provide error messages in the returned HTML. Of course, an API would have been nice, but this was what we had to do.)

Not allowing for that is not only naive from a security standpoint, but also non-standard: a client should be allowed to send HTTP by whatever means they wish, and you should respond correctly. That includes validation.

Server side validation is also important for compatibility - not all users, even if they're using a browser, will have JavaScript enabled.

Addendum - December 2016

There are some validations that can't even be properly done in server-side application code, and are utterly impossible in client-side code, because they depend on the current state of the database. For example, "nobody else has registered that username", or "the blog post you're commenting on still exists", or "no existing reservation overlaps the dates you requested", or "your account balance still has enough to cover that purchase." Only the database can reliably validate data which depends on related data. Developers regularly screw this up, but PostgreSQL provides some good solutions.