If you’ve read many of my previous posts, you have probably seen me use polyfills (i.e. CRUD Operations for SharePoint Docs Using Fetch and REST), to patch older browsers with modern functionality like fetch. I generally download the polyfill, upload it to SharePoint, and load it on the page as a user custom action. But there is another way to load polyfills, which is generally called a polyfill service. The idea is that you load the polyfill from some external service, which detects your current browser, and loads just enough polyfill to patch your current browser up to some level of specification compatibility (usually like ES5, but you can also generally ask for specific functionality, like fetch and/or Promise). There are some unique problems with loading this kind of polyfill in SharePoint, mostly due to limitations in user custom actions. In this post I’m going to talk about how to load such a polyfill in SharePoint, but first lets talk a little more about polyfill services in general.