js-sys: Add extends to JSON

Part of rustwasm#670

crates/js-sys/src/lib.rs

@@ -3048,8 +3048,12 @@ pub mod WebAssembly {
 // JSON
 #[wasm_bindgen]
 extern "C" {
-
+    /// The `JSON` object contains methods for parsing [JavaScript Object
+    /// Notation (JSON)](https://json.org/) and converting values to JSON. It
+    /// can't be called or constructed, and aside from its two method
+    /// properties, it has no interesting functionality of its own.
     #[derive(Clone, Debug)]
+    #[wasm_bindgen(extends = Object)]
     pub type JSON;
 
     /// The `JSON.parse()` method parses a JSON string, constructing the

crates/js-sys/tests/wasm/JSON.rs

@@ -1,4 +1,4 @@
-use wasm_bindgen::JsValue;
+use wasm_bindgen::prelude::*;
 use wasm_bindgen_test::*;
 use wasm_bindgen::JsCast;
 use js_sys::*;
@@ -82,3 +82,15 @@ fn stringify_error() {
     let err_msg: String = From::from(err.message());
     assert!(err_msg.contains("rust really rocks"));
 }
+
+#[wasm_bindgen_test]
+fn json_extends() {
+    #[wasm_bindgen]
+    extern {
+        #[wasm_bindgen(js_name = JSON)]
+        static json: JSON;
+    }
+
+    assert!(json.is_instance_of::<Object>());
+    let _: &Object = json.as_ref();
+}