How can I conditionally show a field in the node/add page?
By default in Drupal 8 "Seven" is the admin theme. 

(Assuming you're using Seven as your admin theme) In /core/themes/seven/js add/create custom.js file.

custom.js

(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);


Now edit seven.libraries.yml

In the node-form: section add

  js:
    js/custom.js: {}


So it should look like this:

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


PS: Don't forget to clear the cache to see changes.