I would like to know what technologies are used in Whatsapp Web development.
Ps: Question at the research level.
NodeJS is used. On the backend, the server used is Yaws, which is developed in Erlang, one of the first languages completely focused on concurrent applications.
On the front, there is no specific technology. Apparently everything is developed in-house. Note only the use of Material Design, flex-box (when supported), ReactJS, Modernizr (to identify what the browser supports), MomentJS, VelocityJS in the animations.
About editors, etc, I believe there is no specific one. Facebook does not restrict its employees to a specific editor or IDE. Only the test flow and their CI that is mandatory (remember that Facebook bought WhatsApp).