Don't overwrite properties on prototype (#1)

index.js

@@ -19,12 +19,11 @@ const knownProps = [
 ];
 
 module.exports = (fromStream, toStream) => {
-	const toProps = Object.keys(toStream);
 	const fromProps = new Set(Object.keys(fromStream).concat(knownProps));
 
 	for (const prop of fromProps) {
 		// Don't overwrite existing properties
-		if (toProps.indexOf(prop) !== -1) {
+		if (prop in toStream) {
 			continue;
 		}
 

test.js

@@ -15,7 +15,7 @@ test.before(async () => {
 	});
 });
 
-test(async t => {
+test('normal', async t => {
 	const response = await pify(http.get, {errorFirst: false})(server.url);
 	response.unicorn = '🦄';
 	response.getContext = function () {
@@ -29,3 +29,23 @@ test(async t => {
 	t.is(toStream.unicorn, '🦄');
 	t.is(toStream.getContext(), response.getContext());
 });
+
+test('do not overwrite prototype properties', async t => {
+	const response = await pify(http.get, {errorFirst: false})(server.url);
+	response.unicorn = '🦄';
+	response.getContext = function () {
+		return this;
+	};
+	const origOn = response.on;
+	response.on = function (name, handler) {
+		return origOn.call(this, name, handler);
+	};
+
+	const toStream = new stream.PassThrough();
+	m(response, toStream);
+
+	t.is(Object.keys(toStream).indexOf('on'), -1);
+	t.is(toStream.statusCode, 200);
+	t.is(toStream.unicorn, '🦄');
+	t.is(toStream.getContext(), response.getContext());
+});