util: speed up formatting of large arrays/objects

Don't .indexOf() into the keys array. V8 is smart but not so smart that it
knows how to turn the linear scan into a O(1) lookup.

Fixes #3562.

lib/util.js

@@ -175,6 +175,17 @@ function stylizeNoColor(str, styleType) {
 }
 
 
+function arrayToHash(array) {
+  var hash = {};
+
+  array.forEach(function(val, idx) {
+    hash[val] = true;
+  });
+
+  return hash;
+}
+
+
 function formatValue(ctx, value, recurseTimes) {
   // Provide a hook for user-specified inspect functions.
   // Check that value is an object with an inspect function on it
@@ -193,8 +204,12 @@ function formatValue(ctx, value, recurseTimes) {
   }
 
   // Look up the keys of the object.
-  var visibleKeys = Object.keys(value);
+  var keys = Object.keys(value);
-  var keys = ctx.showHidden ? Object.getOwnPropertyNames(value) : visibleKeys;
+  var visibleKeys = arrayToHash(keys);
+
+  if (ctx.showHidden) {
+    keys = Object.getOwnPropertyNames(value);
+  }
 
   // Some type of object without properties can be shortcutted.
   if (keys.length === 0) {
@@ -334,7 +349,7 @@ function formatProperty(ctx, value, recurseTimes, visibleKeys, key, array) {
       str = ctx.stylize('[Setter]', 'special');
     }
   }
-  if (visibleKeys.indexOf(key) < 0) {
+  if (!visibleKeys.hasOwnProperty(key)) {
     name = '[' + key + ']';
   }
   if (!str) {