Doubts about client side validation

Question

I read from one of the blog, for security reasons, it was suggested to do an input check on the server side, and not on the client side.

From the above statement, My mind began to ask then 1) what kind of check can I do with JS on the client side. 2) Is there any rule where any programmer can decide where to check, whether on the client side or on the server side?

I am creating a Java based web application

Answer 1

You can do any checks on the client, but you should never rely on this check.

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The client-side check, however, should be considered a NO check, as clients can disable this check and enter invalid values.

The server should always check the entered data. Client verification is for convenience only.