Validation is client-side only. Minuses?

Question

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Of course, we are no longer living in the Stone Age, but how safe is it to do data validation only on the client side? And in general, what's the point in double validation (both on the server side and on the client side)? Is it possible to get by with mostly client-side data validation?

Answer 1

Either only back, or front and back validation.
At the front, you need it for speed, so as not to send data back and forth and not wait for this time.

Answer 2

if you have a simple letter, then it didn’t go anywhere else, but what if there are some financial transactions?
I change BTC USD at the rate of 1 to 100500 thousand millions, and you have a "bankrupt" sign on your balance

Answer 3

Never trust data coming from a client!