guide: Add section on accessing properties of untyped values

Part of #616

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

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+# Accessing Properties of Untyped JavaScript Values
+
+To read and write arbitrary properties from any untyped JavaScript value
+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.
+
+## 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)
+
+A function that returns the value of a property.
+
+#### Rust Usage
+
+```rust
+let value = js_sys::Reflect::get(&target, &property_key);
+```
+
+#### JavaScript Equivalent
+
+```js
+let value = target[property_key];
+```
+
+## Writing Properties with `js_sys::Reflect::set`
+
+[API documentation for `js_sys::Reflect::set`.](https://rustwasm.github.io/wasm-bindgen/api/js_sys/struct.Reflect.html#method.set)
+
+A function that assigns a value to a property. Returns a boolean that is true if
+the update was successful.
+
+#### Rust Usage
+
+```rust
+js_sys::Reflect::set(&target, &property_key, &value);
+```
+
+#### JavaScript Equivalent
+
+```js
+target[property_key] = value;
+```
+
+## Determining if a Property Exists with `js_sys::Reflect::has`
+
+[API documentation for `js_sys::Reflect::has`.](https://rustwasm.github.io/wasm-bindgen/api/js_sys/struct.Reflect.html#method.has)
+
+The JavaScript `in` operator as function. Returns a boolean indicating whether
+an own or inherited property exists on the target.
+
+#### Rust Usage
+
+```rust
+if js_sys::Reflect::has(&target, &property_key) {
+    // ...
+} else {
+    // ...
+}
+```
+
+#### JavaScript Equivalent
+
+```js
+if (property_key in target) {
+    // ...
+} else {
+    // ...
+}
+```
+
+## But wait — there's more!
+
+See [the `js_sys::Reflect` API documentation][js-sys-reflect] for the full
+listing of JavaScript value reflection and introspection capabilities.
+
+[js-sys-reflect]: https://rustwasm.github.io/wasm-bindgen/api/js_sys/struct.Reflect.html
+[mdn-reflect]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Reflect