Preserve custom properties on the original stream (#8)

Fixes #6

index.js

@@ -2,6 +2,38 @@
 const PassThrough = require('stream').PassThrough;
 const zlib = require('zlib');
 
+// We define these manually to ensure they're always copied
+// even if they would move up the prototype chain
+// https://nodejs.org/api/http.html#http_class_http_incomingmessage
+const knownProps = [
+	'destroy',
+	'setTimeout',
+	'socket',
+	'headers',
+	'trailers',
+	'rawHeaders',
+	'statusCode',
+	'httpVersion',
+	'httpVersionMinor',
+	'httpVersionMajor',
+	'rawTrailers',
+	'statusMessage'
+];
+
+const copyProps = (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) {
+			continue;
+		}
+
+		toStream[prop] = typeof prop === 'function' ? fromStream[prop].bind(fromStream) : fromStream[prop];
+	}
+};
+
 module.exports = res => {
 	// TODO: Use Array#includes when targeting Node.js 6
 	if (['gzip', 'deflate'].indexOf(res.headers['content-encoding']) === -1) {
@@ -11,17 +43,7 @@ module.exports = res => {
 	const unzip = zlib.createUnzip();
 	const stream = new PassThrough();
 
-	// https://nodejs.org/api/http.html#http_class_http_incomingmessage
+	copyProps(res, stream);
-	stream.destroy = res.destroy.bind(res);
-	stream.setTimeout = res.setTimeout.bind(res);
-	stream.socket = res.socket;
-	stream.headers = res.headers;
-	stream.trailers = res.trailers;
-	stream.rawHeaders = res.rawHeaders;
-	stream.statusCode = res.statusCode;
-	stream.httpVersion = res.httpVersion;
-	stream.rawTrailers = res.rawTrailers;
-	stream.statusMessage = res.statusMessage;
 
 	unzip.on('error', err => {
 		if (err.code === 'Z_BUF_ERROR') {

test/test.js

@@ -47,8 +47,18 @@ test.after('cleanup', async () => {
 test('decompress gzipped content', async t => {
 	const res = m(await httpGetP(s.url));
 
-	t.is(typeof res.httpVersion, 'string');
+	t.truthy(res.destroy);
+	t.truthy(res.setTimeout);
+	t.truthy(res.socket);
 	t.truthy(res.headers);
+	t.truthy(res.trailers);
+	t.truthy(res.rawHeaders);
+	t.truthy(res.statusCode);
+	t.truthy(res.httpVersion);
+	t.truthy(res.httpVersionMinor);
+	t.truthy(res.httpVersionMajor);
+	t.truthy(res.rawTrailers);
+	t.truthy(res.statusMessage);
 
 	res.setEncoding('utf8');
 
@@ -76,3 +86,13 @@ test('ignore missing data', async t => {
 
 	t.is(await getStream(res), fixture);
 });
+
+test('preserves custom properties on the stream', async t => {
+	let res = await httpGetP(s.url);
+	res.customProp = '🦄';
+	res = m(res);
+
+	t.is(res.customProp, '🦄');
+
+	res.destroy();
+});