polyfill javascript

1Polyfill
2A polyfill is a piece of code (usually JavaScript on the Web) used to provide modern functionality on older browsers that do not natively support it.
3
4For example, a polyfill could be used to mimic the functionality of a text-shadow in IE7 using proprietary IE filters, or mimic rem units or media queries by using JavaScript to dynamically adjust the styling as appropriate, or whatever else you require.
5
6The reason why polyfills are not used exclusively is for better functionality and better performance. Native implementations of APIs can do more and are faster than polyfills. For example, the Object.create polyfill only contains the functionalities that are possible in a non-native implementation of Object.create.

1A polyfill is a browser fallback, made in JavaScript, that allows functionality you expect to work in modern browsers to work in older browsers, e.g., to support canvas (an HTML5 feature) in older browsers.
2
3It's sort of an HTML5 technique, since it is used in conjunction with HTML5, but it's not part of HTML5, and you can have polyfills without having HTML5 (for example, to support CSS3 techniques you want).
4
5Here's a good post:
6
7http://remysharp.com/2010/10/08/what-is-a-polyfill/
8
9Here's a comprehensive list of Polyfills and Shims:
10
11https://github.com/Modernizr/Modernizr/wiki/HTML5-Cross-browser-Polyfills