Note: As of RFC 5 this attribute is the default for all imported functions. This attribute is largely ignored today and is only retained for backwards compatibility and learning purposes.

The inverse of this attribute, the final attribute is more functionally interesting than structural (as structural is simply the default)

The structural flag can be added to method annotations, indicating that the method being accessed (or property with getters/setters) should be accessed in a structural, duck-type-y fashion. Rather than walking the constructor's prototype chain once at load time and caching the property result, the prototype chain is dynamically walked on every access.

# #![allow(unused_variables)]
#fn main() {
#[wasm_bindgen]
extern "C" {
    type Duck;

    #[wasm_bindgen(method, structural)]
    fn quack(this: &Duck);

    #[wasm_bindgen(method, getter, structural)]
    fn is_swimming(this: &Duck) -> bool;
}
#}

The constructor for the type here, Duck, is not required to exist in JavaScript (it's not referenced). Instead wasm-bindgen will generate shims that will access the passed in JavaScript value's quack method or its is_swimming property.

// Without `structural`, get the method directly off the prototype at load time:
const Duck_prototype_quack = Duck.prototype.quack;
function quack(duck) {
  Duck_prototype_quack.call(duck);
}

// With `structural`, walk the prototype chain on every access:
function quack(duck) {
  duck.quack();
}