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;
 }

tests/all/js_globals/ArrayIterator.rs

@@ -0,0 +1,34 @@
+#![allow(non_snake_case)]
+
+use project;
+
+#[test]
+fn keys() {
+    project()
+        .file("src/lib.rs", r#"
+            #![feature(proc_macro, wasm_custom_section)]
+
+            extern crate wasm_bindgen;
+            use wasm_bindgen::prelude::*;
+            use wasm_bindgen::js;
+
+            #[wasm_bindgen]
+            pub fn get_keys(this: &js::Array) -> js::ArrayIterator {
+                this.keys()
+            }
+
+        "#)
+        .file("test.ts", r#"
+            import * as assert from "assert";
+            import * as wasm from "./out";
+
+            export function test() {
+                let characters = [8, 5, 4, 3, 1, 2]
+                let iterator = characters.keys();
+                let wasmIterator = wasm.get_keys(characters);
+
+                assert.equal(iterator.toString(), wasmIterator.toString());
+            }
+        "#)
+        .test()
+}

tests/all/js_globals/mod.rs

@@ -4,6 +4,7 @@ use super::project;
 
 mod Object;
 mod Array;
+mod ArrayIterator;
 
 #[test]
 #[cfg(feature = "std")]