As a JavaScript developer, you’ve no doubt ran across an HTML5 library and thought the same thing, “Is it really a shim, or is it really a polyfill?” If you’re like me, you stopped, looked at both terms, and then scratched your head… In this article I’ll clarify the definition of a JavaScript shim and a JavaScript polyfill so that you can be an informed developer. Knowing the difference will allow you to better choose the libraries you use.