Shim and Polyfill difference explanation

Polyfill is a kind of shim.
Shim is to encapsulate different APIs into one, such as JQuery's $.ajax package XMLHttpRequest and IE ActiveXObject way to create XHR objects;
Polyfill specifically Shim API is standard , its typical practice is to add window in IE browser. XMLHttpRequest, the internal implementation uses ActiveXObject.
In fact, in order to facilitate the comparison, will be specifically Shim API is not to follow the standard, but their own design. Know Li Jingwei
Reference: What ' s a polyfill

Shim and Polyfill difference explanation