Definition
JavaScript code that implements modern web features on older browsers lacking native support—bridging the gap between progressive enhancement ambitions and legacy user agents.
Why It Matters
The web is a fragmented landscape of old and new. Polyfills allow developers to use the latest, most efficient tools while ensuring that users on legacy systems aren’t left behind, maintaining a universal reach without compromising on modern standards.
Core Concepts
- HTML5 Shiv: Enables IE8 to style HTML5 elements (
<nav>,<article>). - Selectivizr: CSS3 selector support for old Internet Explorer.
- Picturefill: Responsive image (
<picture>,srcset) support for legacy browsers. - Principle: Detect missing feature, inject implementation, then run enhanced code.
// Simple polyfill pattern for Array.prototype.includes
if (!Array.prototype.includes) {
Array.prototype.includes = function(searchElement) {
return this.indexOf(searchElement) !== -1;
};
}