add binding for keys method

src/js.rs

@@ -195,5 +195,15 @@ extern {
     /// http://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/includes
     #[wasm_bindgen(method)]
     pub fn includes(this: &Array, value: JsValue, from_index: i32) -> bool;
+}
 
+#[wasm_bindgen]
+extern {
+    pub type ArrayIterator;
+
+    /// The keys() method returns a new Array Iterator object that contains the keys for each index in the array.
+    ///
+    /// http://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/keys
+    #[wasm_bindgen(method)]
+    pub fn keys(this: &Array) -> ArrayIterator;
 }