examples: Add an example of using duck-typed interfaces

examples/duck-typed-interfaces/src/lib.rs

@@ -0,0 +1,23 @@
+extern crate wasm_bindgen;
+use wasm_bindgen::prelude::*;
+
+/// Here is a duck-typed interface for any JavaScript object that has a `quack`
+/// method.
+///
+/// Note that any attempts to check if an object is a `Quacks` with
+/// `JsCast::is_instance_of` (i.e. the `instanceof` operator) will fail because
+/// there is no JS class named `Quacks`.
+#[wasm_bindgen]
+extern {
+    pub type Quacks;
+
+    #[wasm_bindgen(structural, method)]
+    pub fn quack(this: &Quacks) -> String;
+}
+
+/// Next, we can export a function that takes any object that quacks:
+#[wasm_bindgen]
+pub fn make_em_quack_to_this(duck: &Quacks) {
+    let s = duck.quack();
+    // ...
+}