Polyfill vs Shim
polyfill
A polyfill is code that implements a feature on web browsers that do not natively support the feature. Formally, "a polyfill is a shim for a browser API."
Polyfill 是一块代码，用来为旧浏览器提供它没有原生支持的较新的功能。举例：Babel polyfill features that are missing in your target environment (through a third-party polyfill such as core-js)
shim
A shim is a library that transparently intercepts API calls and changes the arguments passed, handles the operation itself or redirects the operation elsewhere.
在一个旧的环境中模拟出一个新 API ，而且仅靠旧环境中已有的手段实现，以便所有的浏览器具有相同的行为。举例：jQuery 发送Ajax请求，屏蔽了浏览器实现细节。