Improve performance for res.json/res.jsonp in most cases

closes #2900

History.md

@@ -1,6 +1,7 @@
 unreleased
 ==========
 
+  * Improve performance for `res.json`/`res.jsonp` in most cases
   * deps: accepts@~1.3.3
     - Fix including type extensions in parameters in `Accept` parsing
     - Fix parsing `Accept` parameters with quoted equals

lib/response.js

@@ -239,7 +239,7 @@ res.json = function json(obj) {
   var app = this.app;
   var replacer = app.get('json replacer');
   var spaces = app.get('json spaces');
-  var body = JSON.stringify(val, replacer, spaces);
+  var body = stringify(val, replacer, spaces);
 
   // content-type
   if (!this.get('Content-Type')) {
@@ -281,7 +281,7 @@ res.jsonp = function jsonp(obj) {
   var app = this.app;
   var replacer = app.get('json replacer');
   var spaces = app.get('json spaces');
-  var body = JSON.stringify(val, replacer, spaces);
+  var body = stringify(val, replacer, spaces);
   var callback = this.req.query[app.get('jsonp callback name')];
 
   // content-type
@@ -1051,3 +1051,16 @@ function sendfile(res, file, options, callback) {
   // pipe
   file.pipe(res);
 }
+
+/**
+ * Stringify JSON, like JSON.stringify, but v8 optimized.
+ * @private
+ */
+
+function stringify(value, replacer, spaces) {
+  // v8 checks arguments.length for optimizing simple call
+  // https://bugs.chromium.org/p/v8/issues/detail?id=4730
+  return replacer || spaces
+    ? JSON.stringify(value, replacer, spaces)
+    : JSON.stringify(value);
+}