Browser side form check and server side form check

Nowadays web developers do browser side form input check by javascript and server side form input check. Although without browser side form input check everything will be OK, we'd better do it. Because we can save some computing power on the server side. If a web user input some thing wrong, the server application need to check for mistakes, generate a page explain the error, and then the server need to check the input and process the data again.

If we have browser side form input check, the error check process will be done on the browser side computer.

But if we can do form input check at the browser side, why do we need check it again at server side? Because there are hackers, there are bad people. They can do whatever to put something wrong onto the server, to crash the application, to mess up our database. So we need to check the input on the server side again.

However, most of the input will be done by good customers, so with the help of browser side form input check, the server side form input check need only work onece, rather than twice or more.