Overview
Javascript Validation Bypass is usually trivial since the user may disable Javascript entirely or fill in values which legitimately pass validation then alter those values in a client proxy such as Burp Suite after the validation has occured.
Video Tutorials
Discovery Methodology
Set up a client proxy to capture all requests. Observe if validation occurs prior to the request being captured. If so, the validation is occuring in the browser.
Exploitation
Disable Javascript entirely and observe if the page still operates. Alternatively fill in values which legitimately pass client-side validation then alter those values in a client proxy such as Burp Suite after the validation has occured.
Videos
Two Methods to Bypass JavaScript Validation Cross-site Scripting Explained - Part 15: Javascript Validation Bypass How to Bypass Maxlength Restrictions on HTML Input Fields