Intellisense

Unable to determine the accuracy of the completion item

Intellisense is unable to determine the accuracy of the completion item 

/// <reference path="https://appsforoffice.microsoft.com/lib/1/hosted/office.js" /> 

You need to provide the definitions of the types in Office.js to Visual Studio
A local copy gets added to the project automatically
These files can be found in the following folder:

ExcelWebAddin/scripts/office/1/