While polyfills are naughty, ponyfills are pure, just like ponies.

A polyfill is code that adds missing functionality by monkey patching an API. Unfortunately, it usually globally patches built-ins, which affects all code running in the environment. This is especially problematic when a polyfill is not fully spec compliant (which in some cases is impossible), as it could cause very hard to debug bugs and inconsistencies. Or when the spec for a new feature changes and your code depends on behavior that a module somewhere else in the dependency tree polyfills differently. In general, you should not modify API's you don't own.

A ponyfill, in contrast, doesn't monkey patch anything, but instead exports the functionality as a normal module, so you can use it locally without affecting other code.

tl;dr; Polyfills are naughty as they patch native APIs, while ponyfills are pure and don't affect the environment.

Number.isNaN = Number.isNaN || function (value) {
	return value !== value;
}

require('is-nan-polyfill');

Number.isNan(5);

module.exports = Number.isNaN || function (value) {
	return value !== value;
}

var isNanPonyfill = require('is-nan-ponyfill');

isNanPonyfill(5);

Only use the native API in your ponyfill (the Number.isNaN || part) when the spec is stable, to ensure it doesn't have differing behavior depending on the environment.

Search npm.

• Ponyfill definition - Sillicon Valley Dictionary
• Polyfills or Ponyfills? - Pony Foo

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Header based on work by Mary Winkler.