JAVASCRIPT

jquery check if field exist by name

if (! $('input[name="inputName"]').length) {
  //Your code when inputName does not exist! 
}

jquery validate if field exists

if( $('#selector').length ) // use this if you are using id to check
{
     // it exists
}

if( $('.selector').length ) // use this if you are using class to check
{
     // it exists
}