Introduction to validating forms

One of the most practical uses of JavaScript is validating forms. Lets first define what is meant by validating forms. Well, using JavaScript, you can check what a user enters into a form, intercept a form from being submitted and ask the user to retype or fill in the unfilled entries of a form before re-submission. Before, you would have to do this using CGI, which is both something I don't want to touch, and something you don't want to with me? Anyone?