util: improve util.format performance

By manually copying arguments and breaking the try/catch out, we are
able to improve the performance of util.format by 20-100% (depending on
the types).

PR-URL: https://github.com/nodejs/node/pull/5360
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/util.js

@@ -9,39 +9,48 @@ const isError = internalUtil.isError;
 
 var Debug;
 
+function tryStringify(arg) {
+  try {
+    return JSON.stringify(arg);
+  } catch (_) {
+    return '[Circular]';
+  }
+}
+
 const formatRegExp = /%[sdj%]/g;
 exports.format = function(f) {
   if (typeof f !== 'string') {
-    var objects = [];
+    const objects = new Array(arguments.length);
     for (var index = 0; index < arguments.length; index++) {
-      objects.push(inspect(arguments[index]));
+      objects[index] = inspect(arguments[index]);
     }
     return objects.join(' ');
   }
 
   if (arguments.length === 1) return f;
 
-  var i = 1;
+  const len = arguments.length;
-  var args = arguments;
+  const args = new Array(len);
-  var len = args.length;
+  var i;
-  var str = String(f).replace(formatRegExp, function(x) {
+  for (i = 0; i < len; i++) {
+    args[i] = arguments[i];
+  }
+
+  i = 1;
+  var str = f.replace(formatRegExp, function(x) {
     if (x === '%%') return '%';
     if (i >= len) return x;
     switch (x) {
       case '%s': return String(args[i++]);
       case '%d': return Number(args[i++]);
-      case '%j':
+      case '%j': return tryStringify(args[i++]);
-        try {
-          return JSON.stringify(args[i++]);
-        } catch (_) {
-          return '[Circular]';
-        }
         // falls through
       default:
         return x;
     }
   });
-  for (var x = args[i]; i < len; x = args[++i]) {
+  while (i < len) {
+    const x = args[i++];
     if (x === null || (typeof x !== 'object' && typeof x !== 'symbol')) {
       str += ' ' + x;
     } else {