Validations Introduction

Validating User Input with JavaScript

JavaScript validation should be considered an assistance more to your users than to your websites because, as I have already stressed many times, you cannot trust any data submitted to your server, even if it has supposedly been validated with JavaScript. This is because hackers can quite easily simulate your web forms and submit any data of their choosing.
 
Another reason you cannot rely on JavaScript to perform all your input validation is that some users disable JavaScript, or use browsers that don’t support it.
 
So the best types of validation to do in JavaScript are checking that fields have content if they are not to be left empty, ensuring that email addresses conform to the proper format, and ensuring that values entered are within expected bounds.