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

src/string.js

Index: src/string.js
diff --git a/src/string.js b/src/string.js
index cb82c166346fc7ba5491a1e9e5d28e0e9b5a5108..c73ef0697c3334a8bb0a11e741e682b6fb4d9c33 100644
--- a/src/string.js
+++ b/src/string.js
@@ -882,6 +882,37 @@ function StringFromCharCode(code) {
 }
 
 
+// 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;")
@@ -1010,7 +1041,8 @@ function SetUpString() {
     "small", StringSmall,
     "strike", StringStrike,
     "sub", StringSub,
-    "sup", StringSup
+    "sup", StringSup,
+    "startsWith", StringStartsWith
   ));
 }