How do you work with forms in your web applications? I am not talking about RESTful applications, I don't want to build heavy front-end using frameworks like Backbone. For example, I need to add "contact us" form. I need to check data which was filled by user and tell him that his data was sent. Requirements: I want to use AJAX. I want to validate form on back-end side and don't want to duplicate the same code on front-end side. I have my own solution, but it doesn't satisfy me. I make an AJAX request with serialized data on form submit and get response. The next is checking "Content-type" header. html - It means that errors with filling form are exists and response html is form with error labels. - I will replace my form with response html. json and response.error_code == 0 - It means that form was successfully submited. - I will show user notification about success. json and response.error_code != 0 - Something was broken on back-end (like connection with database). other - I display the following message : We have been notified and have started to work with that problem. Please, try it later. The problem of that way is that I can't use it with forms that upload file. What is your practise? What libraries and principles do you use?