This adds new methods to String object:

test/mjsunit/harmony/string-startswith.js

+// Copyright 2013 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
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+// Flags: --harmony-strings
+
+assertEquals(1, String.prototype.startsWith.length);
+
+var testString = "Hello World";
+assertTrue(testString.startsWith(""));
+assertTrue(testString.startsWith("Hello"));
+assertFalse(testString.startsWith("hello"));
+assertFalse(testString.startsWith("Hello World!"));
+assertFalse(testString.startsWith(null));
+assertFalse(testString.startsWith(undefined));
+
+assertTrue("null".startsWith(null));
+assertTrue("undefined".startsWith(undefined));
+
+var georgianUnicodeString = "\u10D0\u10D1\u10D2\u10D3\u10D4\u10D5\u10D6\u10D7";
+assertTrue(georgianUnicodeString.startsWith(georgianUnicodeString));
+assertTrue(georgianUnicodeString.startsWith("\u10D0\u10D1\u10D2"));
+assertFalse(georgianUnicodeString.startsWith("\u10D8"));
+
+assertThrows("String.prototype.startsWith.call(null, 'test')", TypeError);
+assertThrows("String.prototype.startsWith.call(null, null)", TypeError);
+assertThrows("String.prototype.startsWith.call(undefined, undefined)", TypeError);
+
+assertThrows("String.prototype.startsWith.apply(null, ['test'])", TypeError);
+assertThrows("String.prototype.startsWith.apply(null, [null])", TypeError);
+assertThrows("String.prototype.startsWith.apply(undefined, [undefined])", TypeError);
+
+var TEST_INPUT = [{
+  msg: "Empty string", val: ""
+}, {
+  msg: "Number 1234.34", val: 1234.34
+}, {
+  msg: "Integer number 0", val: 0
+}, {
+  msg: "Negative number -1", val: -1
+}, {
+  msg: "Boolean true", val: true
+}, {
+  msg: "Boolean false", val: false
+}, {
+  msg: "Regular expression /\d+/", val: /\d+/
+}, {
+  msg: "Empty array []", val: []
+}, {
+  msg: "Empty object {}", val: {}
+}, {
+  msg: "Array of size 3", val: new Array(3)
+}];
+
+function testNonStringValues() {
+  var i = 0;
+  var l = TEST_INPUT.length;
+
+  for (; i < l; i++) {
+    var e = TEST_INPUT[i];
+    var v = e.val;
+    var s = String(v);
+    assertTrue(s.startsWith(v), e.msg);
+    assertTrue(String.prototype.startsWith.call(v, v), e.msg);
+    assertTrue(String.prototype.startsWith.apply(v, [v]), e.msg);
+  }
+}
+testNonStringValues();
+
+var CustomType = function(value) {
+  this.startsWith = String.prototype.startsWith;
+  this.toString = function() {
+    return String(value);
+  }
+};
+
+function testCutomType() {
+  var i = 0;
+  var l = TEST_INPUT.length;
+
+  for (; i < l; i++) {
+    var e = TEST_INPUT[i];
+    var v = e.val;
+    var o = new CustomType(v);
+    assertTrue(o.startsWith(v), e.msg);
+  }
+}
+testCutomType();
+
+// Test cases found in FF
+assertTrue("abc".startsWith("abc"));
+assertTrue("abcd".startsWith("abc"));
+assertTrue("abc".startsWith("a"));
+assertFalse("abc".startsWith("abcd"));
+assertFalse("abc".startsWith("bcde"));
+assertFalse("abc".startsWith("b"));
+assertTrue("abc".startsWith("abc", 0));
+assertFalse("abc".startsWith("bc", 0));
+assertTrue("abc".startsWith("bc", 1));
+assertFalse("abc".startsWith("c", 1));
+assertFalse("abc".startsWith("abc", 1));
+assertTrue("abc".startsWith("c", 2));
+assertFalse("abc".startsWith("d", 2));
+assertFalse("abc".startsWith("dcd", 2));
+assertFalse("abc".startsWith("a", 42));
+assertFalse("abc".startsWith("a", Infinity));
+assertTrue("abc".startsWith("a", NaN));
+assertFalse("abc".startsWith("b", NaN));
+assertTrue("abc".startsWith("ab", -43));
+assertTrue("abc".startsWith("ab", -Infinity));
+assertFalse("abc".startsWith("bc", -42));
+assertFalse("abc".startsWith("bc", -Infinity));