guide: Add section on working with duck-typed interfaces

examples/duck-typed-interfaces/README.md

@@ -1,7 +1,6 @@
-# Canvas 2D Example
+# Duck-Typed Interfaces Example
 
-This directory is an example of using the `web-sys` crate to draw on a 2D
+This directory is an example of using duck-typed JS interfaces with `wasm-bindgen`.
-canvas.
 
 You can build and run the example with:
 

guide/src/SUMMARY.md

@@ -15,6 +15,7 @@
   - [No ES Modules](./reference/no-esm.md)
   - [Arbitrary Data with Serde](./reference/arbitrary-data-with-serde.md)
   - [Accessing Properties of Untyped JS Values](./reference/accessing-properties-of-untyped-js-values.md)
+  - [Working with Duck-Typed Interfaces](./reference/working-with-duck-typed-interfaces.md)
   - [Command Line Interface](./reference/cli.md)
   - [Supported Types](./reference/types.md)
     - [Imported JavaScript Types](./reference/types/imported-js-types.md)

guide/src/reference/accessing-properties-of-untyped-js-values.md

@@ -5,6 +5,10 @@ regardless if it is an `instanceof` some JavaScript class or not, use [the
 `js_sys::Reflect` APIs][js-sys-reflect]. These APIs are bindings to the
 [JavaScript builtin `Reflect` object][mdn-reflect] and its methods.
 
+You might also benefit from [using duck-typed
+interfaces](./working-with-duck-typed-interfaces.html) instead of working with
+untyped values.
+
 ## Reading Properties with `js_sys::Reflect::get`
 
 [API documentation for `js_sys::Reflect::get`.](https://rustwasm.github.io/wasm-bindgen/api/js_sys/struct.Reflect.html#method.get)

guide/src/reference/working-with-duck-typed-interfaces.md

@@ -0,0 +1,20 @@
+# Working with Duck-Typed Interfaces
+
+Liberal use of [the `structural`
+attribute](./attributes/on-js-imports/structural.html) on imported methods,
+getters, and setters allows you to define duck-typed interfaces. A duck-typed
+interface is one where many different JavaScript objects that don't share the
+same base class in their prototype chain and therefore are not `instanceof` the
+same base can be used the same way.
+
+## Defining a Duck-Typed Interface in Rust
+
+```rust
+{{#include ../../../examples/duck-typed-interfaces/src/lib.rs}}
+```
+
+## JavaScript Usage
+
+```js
+{{#include ../../../examples/duck-typed-interfaces/duck-typed-interfaces.js}}
+```