Book excerpt: Summary CORS in Action introduces Cross-Origin Resource Sharing (CORS) from both the server and the client perspective. It starts with the basics: how to make CORS requests and how to implement CORS on the server. It then explores key details such as performance, debugging, and security. API authors will learn how CORS opens their APIs to a wider range of users. JavaScript developers will find valuable techniques for building rich web apps that can take advantage of APIs hosted anywhere. The techniques described in this book are especially applicable to mobile environments, where browsers are guaranteed to support CORS. Purchase of the print book includes a free eBook in PDF, Kindle, and ePub formats from Manning Publications. About the Book Suppose you need to share some JSON data with another application or service. If everything is hosted on one domain, it's a snap. But if the data is on another domain, the browser's "same-origin" policy stops you cold. CORS is a new web standard that enables safe cross-domain access without complex server-side code. Mastering CORS makes it possible for web and mobile applications to share data simply and securely. CORS in Action introduces CORS from both the server and the client perspective. It starts with making and enabling CORS requests and then explores performance, debugging, and security. You'll learn to build apps that can take advantage of APIs hosted anywhere and how to write APIs that expand your products to a wider range of users. For web developers comfortable with JavaScript. No experience with CORS is assumed. What's Inside CORS from the ground up Serving and consuming cross-domain data Best practices for building CORS APIs When to use CORS alternatives like JSON-P and proxies About the Author Monsur Hossain is an engineer at Google who has worked on API-related projects such as the Google JavaScript Client, the APIs Discovery Service, and CORS support for Google APIs. Table of Contents PART 1 INTRODUCING CORS The Core of CORS Making CORS requests PART 2 CORS ON THE SERVER Handling CORS requests Handling preflight requests Cookies and response headers Best practices PART 3 DEBUGGING CORS REQUESTS Debugging CORS requests APPENDIXES CORS reference Configuring your environment What is CSRF? Other cross-origin techniques