How do I create a computed field? I'm creating a Webform in Drupal 8. On the form, I have two number fields; when a user populates them with values, a third field will display the value of the two field multiplied together. 

How can I create that computed field?

P.S. If the user changes the values in either the fields, I need the value in the computed field to update on the fly.
There is a recipe for How to implement client-side calculations.