In web development, a polyfill (or polyfiller) is downloadable code which provides facilities that are not built into a web browser. It implements technology that a developer expects the browser to provide natively, providing a more uniform API landscape. For example, many features of HTML5 are not supported by versions of Internet Explorer older than version 8 or 9, but can be used by web pages if those pages install a polyfill. Web shims and HTML5 Shivs are related concepts.
Sharp decided upon the term polyfill that can imply filling in missing browser functionality and using any number of techniques (poly can mean “many” in Greek). Polyfilla, a paste used to cover up cracks and holes in walls, was also a visualization that Sharp found fitting for the term. He has received feedback stating that the “word should be changed”, but the term has now grown more popular amongst web developers. Sharp intentionally did not promote the term widely, only using it in specific cases and believes that it received a large amount of exposure after Paul Irish directly referenced the term in a presentation months after its inception and was helped become popular due to Modernizr's “HTML5 shims & polyfill” page. Though the term was coined in 2009, the concept predates the coinage, with server side UI component technologies such as JavaServer Faces offering polyfill capabilities since 2004.
Polyfill differs from a shim, in that it can be removed without any changes to the rest of the code once the un-implemented API it substitutes for is properly included in the browser.
- In IE versions prior to 9, unknown HTML elements like
<nav>would be parsed as empty elements, breaking the page's nesting structure and making those elements impossible to style using CSS. One of the most widely used polyfills, html5shiv exploits another quirk of IE to work around this bug: calling
document.createElement("tagname")for each of the new HTML5 elements, which causes IE to parse them correctly. It also includes basic default styling for those HTML5 elements.
- Though most polyfills target out-of-date browsers, some exist to simply push modern browsers forward a little bit more. Lea Verou's -prefix-free polyfill is such a polyfill, allowing current browsers to recognise the unprefixed versions of several CSS3 properties instead of requiring the developer to write out all the vendor prefixes. It reads the page's stylesheets and replaces any unprefixed properties with their prefixed counterparts recognised by the current browser.
- Possibly one of the most anticipated features of CSS3, Flexible Box Layout (a.k.a. Flexbox) promises to be an extremely powerful tool for laying out interface elements. WebKit and Mozilla engines have supported a preliminary draft syntax for years. Flexie implements support for that same syntax in IE and Opera. However, the draft spec has undergone a drastic revision to a new (and much more powerful) syntax, which is not yet supported by Flexie. Flexie can still be used along with the old syntax, but the developer must make sure they include the new syntax for future browsers as well.
- CSS3 PIE
- PIE ("Progressive Internet Explorer") implements some of the most popular missing CSS3 box decoration properties in IE, including border-radius and box-shadow for IE 8 and below, and linear-gradient backgrounds for IE 9 and below. Invoked as a HTC behavior (a proprietary IE feature), it looks for the unsupported CSS3 properties on specific elements and renders those properties using VML for IE 6-8 and SVG for IE 9. Its rendering is mostly indistinguishable from native browser implementations and it handles dynamic DOM modification well.
- JSON 2
- Douglas Crockford originally wrote json2.js as an API for reading and writing his (then up-and-coming) JSON data format. It became so widely used that browser vendors decided to implement its API natively and turn it into a ''de facto'' standard; json2.js was transformed from a library to a polyfill after the fact.
- FlashCanvas is an implementation of the HTML5 Canvas API using a Flash plug-in. A rare commercial polyfill, it comes in a paid version, as well as a free version, which lacks a few advanced features like shadows.
- John Dyer's MediaElement.js polyfills support for
<audio>elements, including the HTML5 MediaElement API, in older browsers using Flash or Silverlight plug-ins. It also provides an optional media player UI for those elements, which is consistent across all browsers.
- Webshims Lib
- Alexander Farkas's Webshims Lib aggregates many other polyfills together into a single package and conditionally loads only those needed by the visiting browser.