• A shim lets you write the same code across all browsers by implementing a new API in downlevel browsers.
• A polyfill is a shim or collection of shims (and a catchy name).
• A prollyfill is a shim for a proposed API
• A helper helps write cross-browser code where a true API shim/polyfill is not possible.

Note that my general approach to polyfills is not to produce 100% compliant behavior, but to provide a broad subset of functionality so that, where possible, cooperative code can be written to take advantage of new APIs. No assumptions should be made about security or edge cases. It is preferrable to use a shim where it is possible to create one on supported browsers. If not possible, a helper should be used that lets the same code be used in all browsers.

I use these in various pages on my sites; most are by me, or I have at least tweaked them. A more comprehensive list can be found at The All-In-One Entirely-Not-Alphabetical No-Bullshit Guide to HTML5 Fallbacks by Paul Irish.

• See web.js for a bundle of the most common Web polyfills (assumes ES5)
  • Includes: html.js dom.js xhr.js cssom.js timing.js
• And polyfill.js has everything in web.js plus es5.js
• Minified web.min.js and polyfill.min.js are also provided c/o http://javascript-minifier.com/

ECMAScript 5 - Most recent standard, supported by all modern browsers. Frozen.

ECMAScript 6 - Based on nearly complete draft standard. Should be stable apart from bug fixes.

ECMAScript 7 - At the initial proposal/strawman stage. Here there be dragons.

JavaScript 1.X String Extras - ref

• String prototype: trimLeft, trimRight, quote

script - tests - living standard

• document.head (for IE8-)
• 'shiv' of newer HTML elements (section, aside, etc), to fix parsing (for IE8-)
• dataset and data-* attributes spec (for IE8+, not available in IE7-)
  • str = element.dataset[key] - yields undefined if data-key attribute not present
  • element.dataset[key] = str - fails unless data-key attribute already present
• Base64 utility methods (for IE9-)
  • encodedString = window.btoa(binaryString) - Base64 Encode
  • binaryString = window.atob(encodedString) - Base64 Decode

script - tests - living standard

• Selectors (for IE7-) - adapted from Paul Young
  • element = document.querySelector(selector)
  • elementArray = document.querySelectorAll(selector)
• elementArray = document.getElementsByClassName(classNames) (for IE8-)
• Node constants: Node.ELEMENT_NODE, etc (for IE8-)
• DOMException constants: DOMException.INDEX_SIZE_ERR (for IE8-)
• Events (for IE8)
  • Where EventTarget is window, document, or any element:
    • EventTarget.addEventListener(event, handler) - for IE8+
    • EventTarget.removeEventListener(event, handler) - for IE8+
  • Event.target
  • Event.currentTarget
  • Event.eventPhase
  • Event.bubbles
  • Event.cancelable
  • Event.timeStamp
  • Event.defaultPrevented
  • Event.stopPropagation()
  • Event.cancelBubble()
• Non-standard Event helpers for IE7- - adapted from QuirksMode
  • window.addEvent(EventTarget, event, handler)
  • window.removeEvent(EventTarget, event, handler)
• DOMTokenList - classListspec, relListspec
  • tokenList = elem.classList - for IE8+
  • tokenList = elem.relList - for IE8+
  • tokenList.length
  • tokenList.item(index)
  • tokenList.contains(token)
  • tokenList.add(token)
  • tokenList.remove(token)
  • tokenList.toggle(token)
  • Non-standard helpers for IE7-:
    • tokenList = window.getClassList(element)
    • tokenList = window.getRelList(element)

script - tests - living standard

• XMLHttpRequest (for IE6-)
• FormData (for IE9-)

script

• W3C Timing control for script-based animations - demo page
  • id = window.requestAnimationFrame()
  • window.cancelAnimationFrame(id)
• Efficient Script Yielding
  • id = setImmediate(callback, args...)
  • clearImmediate(id)

script - spec

Polyfill for width and height in getBoundingClientRect() in IE8-

script - tests - living standard

var url = new URL(url, base);
var value = url.searchParams.get(name);
var valueArray = url.searchParams.getAll(name);
url.searchParams.append(name, valueOrValues);
url.searchParams.delete(name);

URL objects have properties:

• href, origin, protocol, username, password, host, hostname, port, pathname, search, searchParams, hash

URLSearchParams objects have:

• append(name, value)
• delete(name)
• get(name)
• getAll(name)
• has(name)
• set(name, value)
• [Symbol.iterator]() - if defined

script - demo page - draft spec

// Adds the following properties to each KeyboardEvent:
event.code
event.key
event.location

// You can get a label for the key using:
KeyboardEvent.queryKeyCap(code);

// IE7- only: In your keydown/keyup handler, call this in your handler
// before accessing the `code`, `key`, or `location` properties:
window.identifyKey(keyboardEvent);

more details

script - demo page - spec - uses freegeoip.net

navigator.geolocation.getCurrentPosition(successCallback, errorCallback, options);
var watchId = navigator.geolocation.watchPosition(successCallback, errorCallback, options);
navigator.geolocation.clearWatch(watchId);

Obsolete and Unmaintained Polyfills