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{{other uses|Polyfill (disambiguation)}}
In [[software development]], a '''polyfill''' is code that implements a new standard feature of
Polyfills allow web developers to use an API regardless of whether or not it is supported by a browser, and usually with minimal overhead. Typically they first check if a browser supports an API, and use it if available, otherwise using their own implementation.<ref name="speakingjs">{{cite book|title=Speaking JavaScript |url=http://speakingjs.com/ |author=Luis Ángel Pérez Castillo |year=2019}}</ref><ref>"It typically checks if a browser supports an API. If it doesn’t, the polyfill installs its own implementation. That allows you to use the API in either case."</ref> Polyfills themselves use other, more supported features, and thus different polyfills may be needed for different browsers. The term is also used as a verb: ''polyfilling'' is providing a polyfill for a feature.
==Definition==
The term is a [[neologism]], coined by Remy Sharp, who required a word that meant "replicate an API using JavaScript (or Flash or whatever) if the browser doesn’t have it natively" while co-writing the book ''Introducing HTML5'' in 2009.<ref name="introducing276">{{cite book |title=Introducing HTML5 |author1=Bruce Lawson |author2=Remy Sharp |section=Introducing Polyfills |pages=[https://books.google.com/books?id=a8HQCk4pbQkC&pg=PA276 276–277]}}</ref><ref name="sharp2010">{{cite web |last=Sharp |first=Remy |title=What is a polyfill? |date=8 October 2010 |url=https://remysharp.com/2010/10/08/what-is-a-polyfill/ |access-date=13 January 2012}}</ref> Formally, "a shim is a [[Library (computing)|library]] that brings a new API to an older environment, using only the means of that environment."<ref name="speakingjs"/> Polyfills exactly fit this definition; the term ''shim'' was also used for early polyfills.<ref>{{cite web |quote=This piece of information makes building an HTML5 compatibility shim for IE7 far easier than had previously been assumed.|title=Mistakes, Sadness, Regret |url=https://ln.hixie.ch/?start=1201080691&count=1 |author-link=Ian Hickson |author=Ian Hickson |date=2008-01-23}}</ref> However, to Sharp ''shim'' connoted non-transparent APIs and workarounds, such as [[spacer GIF]]s for layout, sometimes known as <code>shim.gif</code>, and similar terms such as ''[[progressive enhancement]]'' and ''[[graceful degradation]]'' were not appropriate, so he invented a new term.<ref name="sharp2010"/> The term is based on the [[spackling paste|multipurpose filling paste]] brand ''[[Spackling paste#Polyfilla|Polyfilla]],'' a paste used to cover up cracks and holes in walls, and the meaning "fill in holes (in functionality) in many ([[wikt:poly-|poly-]]) ways." The word has since gained popularity, particularly due to its use by [[Paul Irish]] and in [[Modernizr]] documentation.<ref name="sharp2010"/><ref>{{cite web |url=https://github.com/Modernizr/Modernizr/wiki/HTML5-Cross-browser-Polyfills |title=HTML5 Cross browser Polyfills |website=[[GitHub]] |archive-url=https://web.archive.org/web/20100928233437/http://github.com/Modernizr/Modernizr/wiki/HTML5-Cross-browser-Polyfills |archive-date=2010-09-28}}</ref>
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== Examples ==
{{refimprove section |date=July 2023}}
=== core-js ===
core-js<ref>{{Cite web|url=https://github.com/zloirock/core-js|title = Core-js|website = [[GitHub]]|date = 26 October 2021}}</ref> is one of the most popular<ref>{{Cite web|url=https://www.npmtrends.com/core-js-vs-core-js-pure-vs-es5-shim-vs-es6-shim-vs-airbnb-js-shims-vs-polyfill-library-vs-polyfill-service-vs-js-polyfills|title = Airbnb-js-shims vs core-js vs core-js-pure vs es5-shim vs es6-shim vs js-polyfills vs polyfill-library vs polyfill-service | NPM trends}}</ref> JavaScript [[standard library]] polyfills. Includes polyfills for [[ECMAScript]] up to the latest version of the standard: promises, symbols, collections, iterators, typed arrays, many other features, ECMAScript proposals, some cross-platform [[WHATWG]] / [[W3C]] features and proposals like <code>URL</code>. You can load only required features or use it without global namespace pollution. It can be integrated with [[Babel (compiler)|Babel]], which allows it to automatically inject required core-js modules into your code.
===
In IE versions prior to 9, unknown HTML elements like {{tag|section|o}} and {{tag|nav|o}} 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,{{efn|The use of the term ''shiv'' here is a pun or mistake on ''shim.''<ref name="introducing276"/>}} exploits another quirk of IE to work around this bug: calling <code>document.createElement("tagname")</code> for each of the new HTML5 elements, which causes IE to parse them correctly. It also includes basic default styling for those HTML5 elements.
=== -prefix-free ===
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.
=== Selectivizr ===
Keith Clark's Selectivizr is a popular polyfill that makes many CSS3 selectors work in IE 8 and below. It reads the page's stylesheets looking for a number of known CSS3 selectors, then uses a JavaScript selector library to query the document for elements matching those selectors, applying the styles directly to those elements. It supports several JavaScript selector libraries such as [[jQuery]].
=== Flexie ===
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.▼
▲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|''de facto'' standard]]; Since json2.js now implements features native to newer browsers into older browsers, it has become a polyfill instead of a library.
=== es5-shim ===
ECMAScript 5th Edition ("ES5") brings some useful new scripting features, and since they're syntactically compatible with older JavaScript engines they can mostly be polyfilled by patching methods onto built-in JS objects. This es5-shim polyfill does it in two parts: es5-shim.js contains those methods that can be fully polyfilled, and es5-sham.js contains partial implementations of the other methods which rely too much on the underlying engine to work accurately.
=== FlashCanvas ===
FlashCanvas is an implementation of the HTML5 Canvas API using an [[Adobe 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.
=== MediaElement.js ===
John Dyer's MediaElement.js polyfills support for <code><video></code> and <code><audio></code> 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.
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=== BrowserID ===
{{Main articles|Mozilla Persona}}
[[Authentication protocol]] proposed by Mozilla, failed to gain traction.<ref>{{cite web | title = navigator.id | url = https://developer.mozilla.org/en/DOM/navigator.id | publisher = Mozilla Developer Network | date = 30 June 2012 | access-date = 9 July 2012 | archive-date = 3 August 2012 | archive-url = https://web.archive.org/web/20120803093440/https://developer.mozilla.org/en/DOM/navigator.id | url-status = dead }}</ref>
=== 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.
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* {{cite web |url= https://github.com/Modernizr/Modernizr/wiki/HTML5-Cross-browser-Polyfills |title= List of polyfills providing HTML5 facilities |___location= GitHub |publisher= [[Modernizr]] project }}
* {{cite web |url= https://html5please.com/ |title= HTML5 Polyfill List by Feature |editor1-first= Divya |editor1-last= Manian |editor2-first= Paul |editor2-last= Irish |editor-link2= Paul Irish |editor3-first= Tim |editor3-last= Branyen |editor4-first= Connor |editor4-last= Montgomery |editor5-first= Arthur |editor5-last= Verschaeve |display-editors=etal |website= HTML5 Please |quote= The recommendations... represent the collective knowledge of developers who have been deep in the HTML5 trenches }}
* {{cite web |url= https://www.moreonfew.com/what-are-polyfills-in-javascript/ |title= What are PolyFills in Javascript? |date=
{{Use dmy dates|date=August 2024}}
[[Category:Compatibility layers]]
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