If you are developing a modern web-based application, chances are you:
- Need to integrate with services that are not completely under your control (or that reside in a different “origin”).
- Have been confronted by this error message in your browser’s console:
XMLHttpRequest cannot load http://external.service/. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://my.app' is therefore not allowed access.
Every time I need to integrate a web app with some external service or some server-side API I have no complete control over, I bump into this error. Google has not yet provided me with a concise description of the problem or an overview of alternatives to perform Cross-Domain requests, so this post will serve as a personal future reference.