How to cast Map contents into my own types?

[ltxitai]

I want to pass around instances of my own type from Rust to JS, then back inside some Map:

#[wasm_bindgen]
struct Foo {
 ...
}

#[wasm_bindgen]
pub fn get_some_foo(...) -> Foo { ... }

#[wasm_bindgen]
pub fn process_foos(foos: &js_sys::Map) { ... }

then on the JS side I want to have something like:

var map = new Map();
map.set("foo1", new Foo());
map.set("foo2", new Foo();
process_foos(map);

The problem arises when I want to use the contents of the map from the Rust side - looks like I can't dyn_into nor dyn_ref to convert to Foo, even though it is marked as #[wasm_bindgen]:

#[wasm_bindgen]
pub fn process_foos(foos: &js_sys::Map) { 
  let foo: Foo = foos.get(&js_sys::JsString::from("foo1")).dyn_into().unwrap();
}

This gives the following error:

   |     let foo: Foo = foos.get(&js_sys::JsString::from("foo")).dyn_into().unwrap();
   |                                                             ^^^^^^^^ the trait `wasm_bindgen::JsCast` is not implemented for `Foo`
   |
   = help: the following other types implement trait `wasm_bindgen::JsCast`:
             wasm_bindgen::JsValue
             js_sys::Array
             ArrayBuffer
             SharedArrayBuffer
             BigInt
             AbortController
             js_sys::Boolean
             AbortSignal
           and 135 others
note: required by a bound in `dyn_into`

Is this possible? Should I implement something manually for my type for this to work? Should/can I use some other type like std::collections::HashMap?

[63b8e7lmk1jv9c0ovus4hqkcasqgp0ed]

The following example worked for me. There's probably other ways, I don't know if there's a better way.

#[wasm_bindgen]
#[derive(Debug)]
struct BindingExample {
    data: String,
}

#[wasm_bindgen]
extern "C" {
    type CustomMap;

    #[wasm_bindgen(catch, method)]
    fn get(this: &CustomMap, key: String) -> Result<BindingExample, JsValue>;
    #[wasm_bindgen(catch, method)]
    fn set(this: &CustomMap, key: String, v: BindingExample) -> Result<JsValue, JsValue>;
}

fn test() {
    let v = BindingExample {
        data: String::from("hello"),
    };
    let map: JsValue = js_sys::Map::new().into();
    let map: CustomMap = map.into();
    let _ = map.set("v".into(), v);
    let v: BindingExample = map.get("v".into()).unwrap();

    console_log!("{v:?}");
}