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

Nested Syntax

Excel.run(function (context) { 
   return context.sync() 
      .then(function () { 
         return context.sync() 
            .then(function () { 
               return context.sync() 

Invoke Chaining Syntax

This first chaining layout requires you to return 'context.sync()' multiple times.

Excel.run(function (context) { 
   return context.sync() 
      .then(function () { 
         return context.sync(); 
      }) 
      .then(function () { 
         return context.sync(); 

Reference Chaining Syntax

This second chaining layout only requires you to have one return statement.

Excel.run(function (context) { 
   return context.sync() 
      .then(function () { 
      .then(context.sync) 
      .then(function () { 
      .then(context.sync) 

Async Await Syntax

Excel.run(async (context) => { 
   await context.sync(); 
   await context.sync(); 
   await context.sync();