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# `structural`
> **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](final.html) is more functionally interesting than
> `structural` (as `structural` is simply the default)
[RFC 5]: https://rustwasm.github.io/rfcs/005-structural-and-deref.html
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.
```rust
#[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.
```js
// 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();
}
```
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