Allow Body.json() typed result

whatwg-fetch/index.d.ts

@@ -34,7 +34,7 @@ interface Body {
     arrayBuffer(): Promise<ArrayBuffer>;
     blob(): Promise<Blob>;
     formData(): Promise<FormData>;
-    json(): Promise<any>;
+    json<T>(): Promise<T>;
     text(): Promise<string>;
 }
 

whatwg-fetch/whatwg-fetch-tests.ts

@@ -68,4 +68,40 @@ function handlePromise(promise: Promise<Response>) {
 	}).then((text) => {
 		console.log(text);
 	});
-}
+}
+
+function test_Body_json() {
+	interface FooBar {
+		foo: string;
+		bar: string;
+	}
+
+	fetch('http://test.com')
+		.then(response => response.json<FooBar>())
+		.then(fooBar => {
+			console.log(fooBar.foo);
+			console.log(fooBar.bar);
+		});
+
+	fetch('http://test.com')
+		.then(response => <Promise<FooBar>> response.json())
+		.then(fooBar => {
+			console.log(fooBar.foo);
+			console.log(fooBar.bar);
+		});
+
+	fetch('http://test.com')
+		.then(response => response.json() as Promise<FooBar>)
+		.then(fooBar => {
+			console.log(fooBar.foo);
+			console.log(fooBar.bar);
+		});
+
+	fetch('http://test.com')
+		.then(response => response.json())
+		.then(fooBar => {
+			// fooBar is {}, not FooBar
+			//console.log(fooBar.foo);
+			//console.log(fooBar.bar);
+		});
+}