How to add custom js validation on field in suitecrm

The best way to add custom js validation on the field level is to add custom js code in vardefs.php,

You can easily findout vardefs.php in any module. And add the sample code in it on any field on which you need custom validation.

'validation' => 
    array (
      'type' => 'callback',
      'callback' => 'function(formname,nameIndex){if($("#" + nameIndex).val()!=888){add_error_style(formname,nameIndex,"Only 888 is allowed!"); return false;}; return true;}',
    )

Code above is sample code that checks that the field should always have 888 value before submitting the form. You can customize it according to your needs.

Thank you