feat(Map): add Map.size

src/js.rs

@@ -345,6 +345,14 @@ extern {
     /// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map/set
     #[wasm_bindgen(method)]
     pub fn set(this: &Map, key: &JsValue, value: &JsValue) -> Map;
+
+    /// The value of size is an integer representing how many entries 
+    /// the Map object has. A set accessor function for size is undefined; 
+    /// you can not change this property.
+    /// 
+    /// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map/size
+    #[wasm_bindgen(method, getter, structural)]
+    pub fn size(this: &Map) -> Number;
 }
 
 // Math

tests/all/js_globals/Map.rs

@@ -181,4 +181,33 @@ fn set() {
             }
         "#)
         .test()
+}
+
+#[test]
+fn size() {
+    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 map_size(this: &js::Map) -> js::Number {
+                this.size()
+            }
+        "#)
+        .file("test.ts", r#"
+            import * as assert from "assert";
+            import * as wasm from "./out";
+
+            export function test() {
+                const map = new Map();
+                map.set('foo', 'bar');
+                map.set('bar', 'baz');
+                assert.equal(wasm.map_size(map), 2);
+            }
+        "#)
+        .test()
 }