How to have a Conditional Field in node form?
You have the Conditional Fields module that works very well in Drupal 7, unfortunately there is not a Drupal 8 version, you can check this issue to find out its progress. If you want you can test the dev version, but I don't know if it works.

Other module in dev is the Field States UI


  Field States UI allows sites builders with minimal PHP/Dev skills to
  configure the Field States API. This lets you configure a field to for
  example hide if another field has a certain value or hasn't been
  filled. While doing it via PHP can be be more powerful the UI can be
  very handy and in some cases quicker.


In the other hand you can make use of the #states property.

EDIT:
As the OP says in the comments there is another module named Field Formatter Condition that can be used for accomplish this task.