diff --git a/format.js b/format.js
index 16f2346..c79ab4e 100644
--- a/format.js
+++ b/format.js
@@ -8,26 +8,30 @@
 
 var sys = require('sys');
 
+function $args(args) {
+    return Array.prototype.slice.call(args);
+}
+
 exports.extendNativeStrings = function() {
     String.prototype.printf = function(/* ... */) {
-        var args = Array.prototype.slice.call(arguments);
-        args.unshift(this);
+        var args = $args(arguments);
+        if (args[0] !== this) args.unshift(this);
         sys.puts(exports.format.apply(this, args));
     };
     String.prototype.format = function(/* ... */) {
-        var args = Array.prototype.slice.call(arguments);
+        var args = $args(arguments);
         if (args[0] !== this) args.unshift(this);
-        exports.format.apply(this, args);
+        return exports.format.apply(this, args);
     };
 };
 
 exports.printf = function(/* ... */) {
-    sys.puts(exports.format.apply(this, Array.prototype.slice.call(arguments)));
+    sys.puts(exports.format.apply(this, arguments));
 };
 
 exports.format = function(format) {
     var argIndex = 1 // skip initial format argument
-      , args = Array.prototype.slice.call(arguments)
+      , args = $args(arguments)
       , i = 0
       , n = format.length
       , result = ''
diff --git a/test_format.js b/test_format.js
index d0f8752..e2c7cec 100644
--- a/test_format.js
+++ b/test_format.js
@@ -1,5 +1,25 @@
-var format = require('./format');
+var sys = require('sys')
+  , format = require('./format')
+  ;
 format.extendNativeStrings();
+
+sys.puts('Testing printf:');
 'hello'.printf();
+sys.puts('(expected "hello")');
 'hello %s'.printf('sami');
+sys.puts('(expected "hello sami")');
 'b: %b\nc: %c\nd: %d\nf: %f\no: %o\ns: %s\nx: %x\nX: %X'.printf(42, 65, 42*42, 42*42*42/1000000000, 255, 'sami', 0xfeedface, 0xc0ffee);
+sys.puts('(expected "b: 101010\nc: A\nd: 1764\nf: 0.000074\no: 0377\ns: sami\nx: 0xfeedface\nX: 0xC0FFEE")');
+sys.puts('(passed if the output looks ok)');
+
+function assertEqual(a, b) {
+    if (a !== b) throw new Error('assertion failed, ' + a + ' !== ' + b);
+}
+
+sys.puts('Testing format:');
+assertEqual('hello'.format(), 'hello');
+assertEqual('hello %s'.format('sami'), 'hello sami');
+assertEqual('b: %b\nc: %c\nd: %d\nf: %f\no: %o\ns: %s\nx: %x\nX: %X'.format(42, 65, 42*42, 42*42*42/1000000000, 255, 'sami', 0xfeedface, 0xc0ffee), "b: 101010\nc: A\nd: 1764\nf: 0.000074\no: 0377\ns: sami\nx: 0xfeedface\nX: 0xC0FFEE");
+sys.puts('(pass)');
+
+sys.puts('all passed');
\ No newline at end of file