Polyfill

A polyfill (poly as in "many" and fill as in "filling in gaps") is a piece of code that provides modern functionality on older browsers or environments that don't natively support it.

For example, a polyfill for the Array.prototype.includes() method:

if (!Array.prototype.includes) {
    Array.prototype.includes = function(searchElement, fromIndex) {
        // implementation
    };
}