Implemented String.prototype.startsWith as per ES6 draft 07-15-13, section 15.5.3.22

src/string.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 864 matching lines @@
   var two_byte = %NewString(n - i, NEW_TWO_BYTE_STRING);
   for (var j = 0; i < n; i++, j++) {
     var code = %_Arguments(i);
     if (!%_IsSmi(code)) code = ToNumber(code) & 0xffff;
     %_TwoByteSeqStringSetChar(two_byte, j, code);
   }
   return one_byte + two_byte;
 }
 
 
+// ES6 draft 07-15-13, section 15.5.3.22
+function StringStartsWith(searchString /* position */) { // length == 1
+  if (IS_NULL_OR_UNDEFINED(this) && !IS_UNDETECTABLE(this)) {
+    throw MakeTypeError("called_on_null_or_undefined",
+                        ["String.prototype.startsWith"]);
+  }
+
+  var s = TO_STRING_INLINE(this);
+  var s_len = s.length;
+  var ss = TO_STRING_INLINE(searchString);
+  var ss_len = ss.length;
+
+  if (ss_len > 0) {
+    var pos = 0;
+    if (%_ArgumentsLength() > 1) {
+      pos = %_Arguments(1);  // position
+      pos = TO_INTEGER(pos);
+    }
+
+    var start = $Math.min($Math.max(pos, 0), s_len);
+    if (ss_len + start > s_len) {
+      return false;
+    }
+
+    return %StringIndexOf(s, ss, start) === start;
+  }
+
+  return true;
+}
+
+
 // Helper function for very basic XSS protection.
 function HtmlEscape(str) {
   return TO_STRING_INLINE(str).replace(/</g, "&lt;")
                               .replace(/>/g, "&gt;")
                               .replace(/"/g, "&quot;")
                               .replace(/'/g, "&#039;");
 }
 
 
 // Compatibility support for KJS.
@@ skipping 108 matching lines @@
     "fontcolor", StringFontcolor,
     "fontsize", StringFontsize,
     "big", StringBig,
     "blink", StringBlink,
     "bold", StringBold,
     "fixed", StringFixed,
     "italics", StringItalics,
     "small", StringSmall,
     "strike", StringStrike,
     "sub", StringSub,
-    "sup", StringSup
+    "sup", StringSup,
+    "startsWith", StringStartsWith
   ));
 }
 
 SetUpString();