http: cleanup _http_common.js

Remove constant deep property access by storing a reference,
use more const, make some conditionals stricter.

PR-URL: #20126
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

lib/_http_common.js

@@ -65,12 +65,12 @@ function parserOnHeadersComplete(versionMajor, versionMinor, headers, method,
   const parser = this;
   const { socket } = parser;
 
-  if (!headers) {
+  if (headers === undefined) {
     headers = parser._headers;
     parser._headers = [];
   }
 
-  if (!url) {
+  if (url === undefined) {
     url = parser._url;
     parser._url = '';
   }
@@ -80,64 +80,61 @@ function parserOnHeadersComplete(versionMajor, versionMinor, headers, method,
                                  socket.server[kIncomingMessage]) ||
                                  IncomingMessage;
 
-  parser.incoming = new ParserIncomingMessage(socket);
-  parser.incoming.httpVersionMajor = versionMajor;
-  parser.incoming.httpVersionMinor = versionMinor;
-  parser.incoming.httpVersion = `${versionMajor}.${versionMinor}`;
-  parser.incoming.url = url;
-  parser.incoming.upgrade = upgrade;
+  const incoming = parser.incoming = new ParserIncomingMessage(socket);
+  incoming.httpVersionMajor = versionMajor;
+  incoming.httpVersionMinor = versionMinor;
+  incoming.httpVersion = `${versionMajor}.${versionMinor}`;
+  incoming.url = url;
+  incoming.upgrade = upgrade;
 
   var n = headers.length;
 
   // If parser.maxHeaderPairs <= 0 assume that there's no limit.
   if (parser.maxHeaderPairs > 0)
     n = Math.min(n, parser.maxHeaderPairs);
 
-  parser.incoming._addHeaderLines(headers, n);
+  incoming._addHeaderLines(headers, n);
 
   if (typeof method === 'number') {
     // server only
-    parser.incoming.method = methods[method];
+    incoming.method = methods[method];
   } else {
     // client only
-    parser.incoming.statusCode = statusCode;
-    parser.incoming.statusMessage = statusMessage;
+    incoming.statusCode = statusCode;
+    incoming.statusMessage = statusMessage;
   }
 
-  return parser.onIncoming(parser.incoming, shouldKeepAlive);
+  return parser.onIncoming(incoming, shouldKeepAlive);
 }
 
 // XXX This is a mess.
 // TODO: http.Parser should be a Writable emits request/response events.
 function parserOnBody(b, start, len) {
-  var parser = this;
-  var stream = parser.incoming;
+  const stream = this.incoming;
 
   // if the stream has already been removed, then drop it.
-  if (!stream)
+  if (stream === null)
     return;
 
-  var socket = stream.socket;
-
   // pretend this was the result of a stream._read call.
   if (len > 0 && !stream._dumped) {
     var slice = b.slice(start, start + len);
     var ret = stream.push(slice);
     if (!ret)
-      readStop(socket);
+      readStop(this.socket);
   }
 }
 
 function parserOnMessageComplete() {
-  var parser = this;
-  var stream = parser.incoming;
+  const parser = this;
+  const stream = parser.incoming;
 
-  if (stream) {
+  if (stream !== null) {
     stream.complete = true;
     // Emit any trailing headers.
-    var headers = parser._headers;
-    if (headers) {
-      parser.incoming._addHeaderLines(headers, headers.length);
+    const headers = parser._headers;
+    if (headers.length) {
+      stream._addHeaderLines(headers, headers.length);
       parser._headers = [];
       parser._url = '';
     }
@@ -151,8 +148,8 @@ function parserOnMessageComplete() {
 }
 
 
-var parsers = new FreeList('parsers', 1000, function() {
-  var parser = new HTTPParser(HTTPParser.REQUEST);
+const parsers = new FreeList('parsers', 1000, function() {
+  const parser = new HTTPParser(HTTPParser.REQUEST);
 
   parser._headers = [];
   parser._url = '';