Improve util.format() compatibility with browser.

Fixes #1434.

doc/api/util.markdown

@@ -17,9 +17,10 @@ argument. Supported placeholders are:
 * `%j` - JSON.
 * `%%` - single percent sign (`'%'`). This does not consume an argument.
 
-If the placeholder does not have a corresponding argument, `undefined` is used.
+If the placeholder does not have a corresponding argument, the placeholder is
+not replaced.
 
-    util.format('%s:%s', 'foo'); // 'foo:undefined'
+    util.format('%s:%s', 'foo'); // 'foo:%s'
 
 If there are more arguments than placeholders, the extra arguments are
 converted to strings with `util.inspect()` and these strings are concatenated,

lib/util.js

@@ -34,7 +34,9 @@ exports.format = function(f) {
 
   var i = 1;
   var args = arguments;
+  var len = args.length;
   var str = String(f).replace(formatRegExp, function(x) {
+    if (i >= len) return x;
     switch (x) {
       case '%s': return String(args[i++]);
       case '%d': return Number(args[i++]);
@@ -44,7 +46,7 @@ exports.format = function(f) {
         return x;
     }
   });
-  for (var len = args.length, x = args[i]; i < len; x = args[++i]) {
+  for (var x = args[i]; i < len; x = args[++i]) {
     if (x === null || typeof x !== 'object') {
       str += ' ' + x;
     } else {

test/simple/test-util-format.js

@@ -47,3 +47,12 @@ assert.equal(util.format('%j', '42'), '"42"');
 
 assert.equal(util.format('%%s%s', 'foo'), '%sfoo');
 
+assert.equal(util.format('%s'), '%s');
+assert.equal(util.format('%s', undefined), 'undefined');
+assert.equal(util.format('%s', 'foo'), 'foo');
+assert.equal(util.format('%s:%s'), '%s:%s');
+assert.equal(util.format('%s:%s', undefined), 'undefined:%s');
+assert.equal(util.format('%s:%s', 'foo'), 'foo:%s');
+assert.equal(util.format('%s:%s', 'foo', 'bar'), 'foo:bar');
+assert.equal(util.format('%s:%s', 'foo', 'bar', 'baz'), 'foo:bar baz');
+