RequestContext

Introduced in 2014 March for Office 2016.
The RequestContext object facilitates requests to the Office application.
This uses application specific APIs.
Because the Add-in and the Office application run in two different processes, request context is required to get access to the Office application from the add-in.
The JavaScript object declared is a proxy object for the real object until the add-in and Office application are synchronized.
A request context is created as shown and this object has two methods.

var context = new Excel.RequestContext(); 

context.sync - (Previously executeAsync when in preview). Submits the request queue to the office application and returns a promise object, which can be used for chaining further actions
context.load - Fills the proxy object with objects and properties that need to be loaded.
context.set -

Async / Await