Initial javascript shim working

.gitignore

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+/target
+/Cargo.lock

Cargo.toml

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+[package]
+name = "web-component-rs"
+version = "0.1.0"
+edition = "2021"
+author = "Jeremy Wall <jeremy@marzhillstudios.com>"
+
+[lib]
+crate-type = ["cdylib", "rlib"]
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies.wasm-bindgen-test]
+version = "0.3"
+
+[dependencies.wasm-bindgen]
+version = "= 0.2.81"
+
+[dependencies.js-sys]
+version = "0.3"
+
+[dependencies.web-sys]
+version = "0.3"
+features = [
+    "CustomElementRegistry",
+    "Document",
+    #"DocumentFragment",
+    "KeyboardEvent",
+    "Event",
+    "EventTarget",
+    "Element",
+    "HtmlBaseElement",
+    "HtmlElement",
+    "HtmlTemplateElement",
+    "HtmlSlotElement",
+    "Node",
+    "ShadowRoot",
+    "ShadowRootInit",
+    "ShadowRootMode",
+    "Window",
+]

src/lib.rs

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+use js_sys::Function;
+use wasm_bindgen::JsValue;
+use web_sys::window;
+
+type Result<T> = std::result::Result<T, JsValue>;
+
+pub trait CustomElementImpl {
+    fn class_name() -> &'static str;
+    fn element_name() -> &'static str;
+}
+
+pub fn define_web_component<T: CustomElementImpl>() -> Result<JsValue> {
+    let body = format!(
+        "class {name} extends HTMLElement {{
+    constructor() {{
+        super();
+    }}
+}}
+customElements.define(\"{element_name}\", {name});
+var element = customElements.get(\"{element_name}\");
+return element;",
+        name = T::class_name(),
+        element_name = T::element_name(),
+    );
+    let fun = Function::new_no_args(&body);
+    Ok(fun.call0(&window().unwrap())?)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use wasm_bindgen::{prelude::*, JsCast};
+    use wasm_bindgen_test::wasm_bindgen_test;
+    wasm_bindgen_test::wasm_bindgen_test_configure!(run_in_browser);
+
+    pub struct MyElementImpl();
+
+    impl CustomElementImpl for MyElementImpl {
+        fn class_name() -> &'static str {
+            "MyElement"
+        }
+
+        fn element_name() -> &'static str {
+            "my-element"
+        }
+    }
+
+    #[wasm_bindgen]
+    extern "C" {
+        #[wasm_bindgen(js_namespace = console, js_name = log)]
+        pub fn log(message: String);
+        #[wasm_bindgen(js_namespace = console, js_name = log)]
+        pub fn log_with_val(message: String, val: &JsValue);
+    }
+
+    #[wasm_bindgen_test]
+    fn test_component_definition() {
+        let obj = define_web_component::<MyElementImpl>().expect("Failed to define web component");
+        let fun = obj.dyn_ref::<Function>().unwrap();
+        assert_eq!(fun.name(), MyElementImpl::class_name());
+
+        let element = window()
+            .unwrap()
+            .document()
+            .unwrap()
+            .create_element(MyElementImpl::element_name())
+            .unwrap();
+        assert_eq!(
+            element.tag_name().to_uppercase(),
+            MyElementImpl::element_name().to_uppercase()
+        );
+    }
+}