Client-Side Validation When you supply a client-side script (usually in the form of JavaScript) in the page that is posted to the user’s browser to perform validations on the data entered in the form before the form is posted back to the originating server, at that time client-side validation has occurred.
Client-side validation is quick and responsive for the user. It is something that users wants to work with. If something is wrong with the form, using client-side validation ensures that the user knows about it as soon as possible. The reason for this is that the client-side validation, if called properly, executes before the form is posted back to the server.
Client-side validation is the more insecure form of validation because When a page is generated in user’s browser, then a user can look at the code of the page quite easily simply by right-clicking his mouse in the browser and selecting View Code. When he or she does so, in addition to seeing the HTML code for the page, all of the JavaScript that is associated with the page will also be viewed.