The OneDrive for Business and Sites APIs now have cross-origin resource sharing (CORS) support and Mail/Calendar/Contacts are coming soon. As more Office 365 API endpoints come online for services they will also support this including Office Graph, Yammer, Video Portal, Skype and content services.
Developers now have the choice to call the Office 365 APIs from the server-side or client-side. This means developers can write single page applications, which have a better user experience and better performance, because they don’t have to proxy all their calls through a server-side façade service.
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