A polyfill is a piece of code, often written in JavaScript, that provides functionality on web browsers that do not natively support that feature. The term "polyfill" is a combination of "poly-", meaning "many", and "fill", which represents the idea of "filling in" gaps in functionality. Essentially, polyfills enable web developers to use modern web features while maintaining compatibility with older browsers.
For example, if a developer wants to use a newer JavaScript API or CSS property that isn't supported in an older browser, they can include a polyfill that detects whether the browser supports that feature. If it doesn't, the polyfill will implement the feature using JavaScript or other means so that the developer can use the feature as if it were natively supported.
The concept of polyfills is particularly important in web development because of the wide range of browsers and versions in use, each with varying levels of support for modern web standards. By using polyfills, developers can write code based on the latest standards without excluding users on older browsers.
Polyfills differ from shims in that they aim to replicate the exact API of the feature they're polyfilling, allowing developers to use the feature as intended. If the polyfill is removed, the code would continue to work without any changes, provided the user's browser supports the feature natively.
The term was coined by Remy Sharp, who needed a word to describe the process of emulating an API using JavaScript when the browser does not support it. The analogy is drawn from the British brand Polyfilla, which is a spackling paste used to fill holes in walls, just as polyfill...
