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.