Inheritance in web-sys

Inheritance between JS classes is the bread and butter of how the DOM works on the web, and as a result it's quite important for web-sys to provide access to this inheritance hierarchy as well! There are few ways you can access the inheritance hierarchy when using web-sys.

Accessing parent classes using Deref

Like smart pointers in Rust, all types in web_sys implement Deref to their parent JS class. This means, for example, if you have a web_sys::Element you can create a web_sys::Node from that implicitly:

# #![allow(unused_variables)]
#fn main() {
let element: &Element = ...;

element.append_child(..); // call a method on `Node`

method_expecting_a_node(&element); // coerce to `&Node` implicitly

let node: &Node = &element; // explicitly coerce to `&Node`

Using Deref allows ergonomic transitioning up the inheritance hierarchy to the parent class and beyond, giving you access to all the methods using the . operator.

Accessing parent classes using AsRef

In addition to Deref, the AsRef trait is implemented for all types in web_sys for all types in the inheritance hierarchy. For example for the HtmlAnchorElement type you'll find:

# #![allow(unused_variables)]
#fn main() {
impl AsRef<HtmlElement> for HtmlAnchorElement
impl AsRef<Element> for HtmlAnchorElement
impl AsRef<Node> for HtmlAnchorElement
impl AsRef<EventTarget> for HtmlAnchorElement
impl AsRef<Object> for HtmlAnchorElement
impl AsRef<JsValue> for HtmlAnchorElement

You can use .as_ref() to explicitly get a reference to any parent class from from a type in web_sys. Note that because of the number of AsRef implementations you'll likely need to have type inference guidance as well.

Accessing child clases using JsCast

Finally the wasm_bindgen::JsCast trait can be used to implement all manner of casts between types. It supports static unchecked casts between types as well as dynamic runtime-checked casts (using instanceof) between types.

More documentation about this can be found on the trait itself