Awesome

Use ponyfill.com for linking here.

Pony pureness, really?

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

How are ponyfills better than polyfills?

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.

Polyfill

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

import 'is-nan-polyfill';

Number.isNaN(5);

Ponyfill

export default function isNaN(value) {
	return value !== value;
};

import isNanPonyfill from 'is-nan-ponyfill';

isNanPonyfill(5);

Ponyfills should avoid using native APIs, because potential bugs or differences in the native APIs will make such a ponyfill less robust (therefore defeating one of its main purposes). There are important exceptions, such as when:

• There is no way to implement some of the ponyfill without native APIs.
• Reimplementing native parts would have a large cost (e.g. performance or code size).

In such cases, it's still valuable for the ponyfill to minimize any assumptions about the underlying environment.

Where can I find ponyfills?

Search npm.

How do I make a ponyfill?

• Read the specification or source code of the feature you want to ponyfill.
• Initialize an npm package.
• Write some tests to ease writing the ponyfill logic.
• Link to documentation about the feature in your readme. Example.
• Link to https://ponyfill.com in your readme. Example.
• Add ponyfill to the keywords section in package.json.
• Publish!

Resources

• Ponyfill definition - Silicon Valley Dictionary
• Polyfills or Ponyfills? - Pony Foo
• Polyfill, Ponyfill and Prollyfill - Kikobeats

License

To the extent possible under law, Sindre Sorhus has waived all copyright and related or neighboring rights to this work.

Header based on work by Mary Winkler.