(function ($) {

   $('#edit-body-wrapper').hide();

   (function Forever(){

     var title = $('#edit-title-0-value').val();

     if (title != '') {
       $('#edit-body-wrapper').slideDown(1500);
     }
     // slides down and shows the body field in 1.5 secs if the title field is not empty

     else if (title == '') {
       $('#edit-body-wrapper').slideUp(1500);
         // slides up and hides the body field in 1.5 secs if title is empty
     }

   setTimeout(Forever, 1);

   })();

})(jQuery);

js:
    js/custom.js: {}

node-form:
  version: VERSION
  css:
    layout:
      css/layout/node-add.css: {}
  dependencies:
    - node/form
  js:
    js/custom.js: {}