"As we have said in the past, we are listening on all the channels we encourage you to participate on (UserVoice, Yammer, StackOverflow, Twitter)," Jeremy Thake, Office 365 product marketing manager, said in a blog post. "CORS support for the Office 365 APIs was taken as a high priority based on this feedback and we appreciate the communities support in providing the justifications for this. We encourage you to continue to provide this feedback for future feature requests."
Client-side calls result in developers writing single-page applications that interact with Office 365 APIs. Calls no longer need to proxy through a server-side service. Microsoft anticipates the offering will lead to a better user experience and better performance. CORS support enables Web clients to request back-end resources from other Web clients in an entirely different domain.
To help developers get started, Microsoft published documentation specifically on CORS support. Available documentation also includes code samples and a series of hands-on labs for building apps. One example Microsoft provided, the Expense Manager, was originally written in ASP.NET (server side) but has now been completely refactored from the client side. This example gives developers an example of a complete transition and the corresponding benefits.