Press F12 (in most modern browsers) to open the developer's tools.
Click on button, submit form, or reload page to trigger the error again.
Check under the "console" tab - It will show the exact file and line where the error occured.
Error messages give you a good hint on what went wrong.
E.G. Uncaught SyntaxError: Unexpected token ‘}’ = Missing a } somewhere.
E.G. Uncaught ReferenceError: numa is not defined = Variable is not defined, or you made a typo mistake.
Can't fix the problem? There are plenty of places you can ask for help.