Firefox 18+ supports String#startsWith, String#endsWith

src/prototype/lang/string.js

@@ -770,33 +770,47 @@ Object.extend(String.prototype, (function() {
   }
 
   /**
-   *  String#startsWith(substring) -> Boolean
+   *  String#startsWith(substring[, position]) -> Boolean
+   *  - substring (String): The characters to be searched for at the start of this string.
+   *  - [position] (Number): The position in this string at which to begin searching for `substring`; defaults to 0.
    *
    *  Checks if the string starts with `substring`.
    *  
    *  ##### Example
    *  
    *      'Prototype JavaScript'.startsWith('Pro');
    *      //-> true
+   *      'Prototype JavaScript'.startsWith('Java', 10);
+   *      //-> true
   **/
-  function startsWith(pattern) {
+  function startsWith(pattern, position) {
+    position = Object.isNumber(position) ? position : 0;
     // We use `lastIndexOf` instead of `indexOf` to avoid tying execution
     // time to string length when string doesn't start with pattern.
-    return this.lastIndexOf(pattern, 0) === 0;
+    return this.lastIndexOf(pattern, position) === position;
   }
 
   /**
-   *  String#endsWith(substring) -> Boolean
+   *  String#endsWith(substring[, position]) -> Boolean
+   *  - substring (String): The characters to be searched for at the end of this string.
+   *  - [position] (Number): Search within this string as if this string were only this long;
+   *    defaults to this string's actual length, clamped within the range established by this string's length.
    *
    *  Checks if the string ends with `substring`.
    *  
    *  ##### Example
    *  
    *      'slaughter'.endsWith('laughter')
    *      // -> true
+   *      'slaughter'.endsWith('laugh', 6)
+   *      // -> true
   **/
-  function endsWith(pattern) {
-    var d = this.length - pattern.length;
+  function endsWith(pattern, position) {
+    pattern = String(pattern);
+    position = Object.isNumber(position) ? position : this.length;
+    if (position < 0) position = 0;
+    if (position > this.length) position = this.length;
+    var d = position - pattern.length;
     // We use `indexOf` instead of `lastIndexOf` to avoid tying execution
     // time to string length when string doesn't end with pattern.
     return d >= 0 && this.indexOf(pattern, d) === d;
@@ -878,8 +892,9 @@ Object.extend(String.prototype, (function() {
     isJSON:         isJSON,
     evalJSON:       NATIVE_JSON_PARSE_SUPPORT ? parseJSON : evalJSON,
     include:        include,
-    startsWith:     startsWith,
-    endsWith:       endsWith,
+    // Firefox 18+ supports String.prototype.startsWith, String.prototype.endsWith
+    startsWith:     String.prototype.startsWith || startsWith,
+    endsWith:       String.prototype.endsWith || endsWith,
     empty:          empty,
     blank:          blank,
     interpolate:    interpolate

test/unit/string_test.js

@@ -430,6 +430,11 @@ new Test.Unit.Runner({
     this.assert(!'hello world'.startsWith('bye'));
     this.assert(!''.startsWith('bye'));
     this.assert(!'hell'.startsWith('hello'));
+
+    var str = "To be, or not to be, that is the question";
+    this.assert(str.startsWith("To be"), 'str.startsWith("To be")');
+    this.assert(!str.startsWith("not to be"), 'str.startsWith("not to be")');
+    this.assert(str.startsWith("not to be", 10), 'str.startsWith("not to be", 10)');
   },
 
   testEndsWith: function() {
@@ -439,6 +444,25 @@ new Test.Unit.Runner({
     this.assert(!''.endsWith('planet'));
     this.assert('hello world world'.endsWith(' world'));
     this.assert(!'z'.endsWith('az'));
+
+    var str = "To be, or not to be, that is the question";
+    this.assert(str.endsWith("question"), 'str.endsWith("question")');
+    this.assert(!str.endsWith("to be"), 'str.endsWith("to be")');
+    this.assert(str.endsWith("to be", 19), 'str.endsWith("to be", 19)');
+
+    str = "12345";
+    this.assert(str.endsWith("5"));
+    this.assert(str.endsWith("5", 6));
+    this.assert(str.endsWith("5", 5));
+    this.assert(!str.endsWith("5", 4));
+    this.assert(!str.endsWith("5", 1));
+    this.assert(!str.endsWith("5", 0));
+
+    this.assert(str.endsWith("1", 1));
+    this.assert(!str.endsWith("1", 0));
+    this.assert(!str.endsWith("1", -1));
+
+    this.assert(str.endsWith("", 0));
   },
 
   testBlank: function() {
@@ -551,4 +575,4 @@ new Test.Unit.Runner({
     this.assertIdentical(false, 'false'.evalJSON());
     this.assertEqual('"', '"\\""'.evalJSON());
   }
-});
+});