Inheritance - The `wasm-bindgen` Guide (original) (raw)

The `wasm-bindgen` Guide

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)]
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 inweb_sys for all types in the inheritance hierarchy. For example for theHtmlAnchorElement type you'll find:

#![allow(unused)]
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 AsRefimplementations you'll likely need to have type inference guidance as well.

Accessing child classes 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.