# Prototype polyfills

The prototype infrastructure makes functions available in modern browsers available to older browsers lacking the native support.

Polyfills should only extend JavaScript and BOM (Browser Object Model). Polyfills that extend the DOM (Document Object Model), i.e. the Element interface, are to be avoided, as they prove difficult in multi-document scenarios (e.g. <iframe>, <object>) and may not be made available on all inheriting interfaces (e.g. SVGElement).

• prototype/window.customevent.js polyfills the CustomEvent constructor (but returns it rather than overwriting window.CustomEvent in Internet Explorer)
• prototype/window.requestanimationframe.js polyfills the requestAnimationFrame function for older Internet Explorer

Additionally ally.js uses the following third party polyfills:

• CSS.escape to polyfill the CSSOM CSS.escape function

# Contributing

ally.js is not using the babel polyfill because it is too big for the rather limited subset we're using at the moment. This may change in the future, but we're not keen on making this move.

ally.js will not pollute prototypes of built-in methods with anything but proper polyfills for (semi-) standardized methods. If we need to add another polyfill, we'll prefer to use an external dependency over implementing things ourselves.