Type Translations in web-sys

Most of the types specified in WebIDL (the interface definition language for all Web APIs) have relatively straightforward translations into web-sys, but it's worth calling out a few in particular:

• BufferSource and ArrayBufferView - these two types show up in a number of APIs that generally deal with a buffer of bytes. We bind them in web-sys with two different types, js_sys::Object and &mut [u8]. Using js_sys::Object allows passing in arbitrary JS values which represent a view of bytes (like any typed array object), and &mut [u8] allows using a raw slice in Rust. Unfortunately we must pessimistically assume that JS will modify all slices as we don't currently have information of whether they're modified or not.

• Callbacks are all represented as js_sys::Function. This means that all callbacks going through web-sys are a raw JS value. You can work with this by either juggling actual js_sys::Function instances or you can create a Closure<dyn FnMut(...)>, extract the underlying JsValue with as_ref, and then use JsCast::unchecked_ref to convert it to a js_sys::Function.