The Tor Browser / file revision

 // NOTE: Requires testharness.js

 // http://www.w3.org/2008/webapps/wiki/Harness

 // Extension to testharness.js API which avoids logging enormous strings

 // on a coding failure.

 function assert_string_equals(actual, expected, description) {

   // short circuit success case

   if (actual === expected) {

     assert_true(true, description + ": <actual> === <expected>");

     return;

   }

   // length check

   assert_equals(actual.length, expected.length,

                 description + ": string lengths")

   var i, a, b;

   for (i = 0; i < actual.length; i++) {

     a = actual.charCodeAt(i);

     b = expected.charCodeAt(i);

     if (a !== b)

       assert_true(false,

                   description +

                   ": code unit " + i.toString() + " unequal: " +

                   cpname(a) + " != " + cpname(b)); // doesn't return

   }

   // It should be impossible to get here, because the initial

   // comparison failed, so either the length comparison or the

   // codeunit-by-codeunit comparison should also fail.

   assert_true(false, description + ": failed to detect string difference");

 }

 // Inspired by:

 // http://ecmanaut.blogspot.com/2006/07/encoding-decoding-utf8-in-javascript.html

 function encode_utf8(string) {

   var utf8 = unescape(encodeURIComponent(string));

   var octets = new Uint8Array(utf8.length), i;

   for (i = 0; i < utf8.length; i += 1) {

     octets[i] = utf8.charCodeAt(i);

   }

   return octets;

 }

 function decode_utf8(octets) {

   var utf8 = String.fromCharCode.apply(null, octets);

   return decodeURIComponent(escape(utf8));

 }

 // Helpers for test_utf_roundtrip.

 function cpname(n) {

   if (n+0 !== n)

     return n.toString();

   var w = (n <= 0xFFFF) ? 4 : 6;

   return 'U+' + ('000000' + n.toString(16).toUpperCase()).slice(-w);

 }

 function genblock(from, len) {

   var i, j, point, offset;

   var size, block;

   // determine size required:

   //    1 unit   for each point from U+000000 through U+00D7FF

   //    0 units                      U+00D800 through U+00DFFF

   //    1 unit                       U+00E000 through U+00FFFF

   //    2 units                      U+010000 through U+10FFFF

   function overlap(min1, max1, min2, max2) {

     return Math.max(0, Math.min(max1, max2) - Math.max(min1, min2));

   }

   size = (overlap(from, from+len, 0x000000, 0x00D800) +

           overlap(from, from+len, 0x00E000, 0x010000) +

           overlap(from, from+len, 0x010000, 0x110000)*2);

   block = new Uint16Array(size);

   for (i = 0, j = 0; i < len; i++) {

     point = from + i;

     if (0xD800 <= point && point <= 0xDFFF)

       continue;

     else if (point <= 0xFFFF)

       block[j++] = point;

     else {

       offset = point - 0x10000;

       block[j++] = 0xD800 + (offset >> 10);

       block[j++] = 0xDC00 + (offset & 0x3FF);

     }

   }

   return String.fromCharCode.apply(null, block);

 }

 function test_utf_roundtrip () {

   var MIN_CODEPOINT = 0;

   var MAX_CODEPOINT = 0x10FFFF;

   var BLOCK_SIZE = 0x1000;

   var block, block_tag, i, j, encoded, decoded, exp_encoded, exp_decoded;

   var TE_U16LE = new TextEncoder("UTF-16LE");

   var TD_U16LE = new TextDecoder("UTF-16LE");

   var TE_U16BE = new TextEncoder("UTF-16BE");

   var TD_U16BE = new TextDecoder("UTF-16BE");

   var TE_U8    = new TextEncoder("UTF-8");

   var TD_U8    = new TextDecoder("UTF-8");

   for (i = MIN_CODEPOINT; i < MAX_CODEPOINT; i += BLOCK_SIZE) {

     block_tag = cpname(i) + " - " + cpname(i + BLOCK_SIZE - 1);

     block = genblock(i, BLOCK_SIZE);

     // test UTF-16LE, UTF-16BE, and UTF-8 encodings against themselves

     encoded = TE_U16LE.encode(block);

     decoded = TD_U16LE.decode(encoded);

     assert_string_equals(block, decoded, "UTF-16LE round trip " + block_tag);

     encoded = TE_U16BE.encode(block);

     decoded = TD_U16BE.decode(encoded);

     assert_string_equals(block, decoded, "UTF-16BE round trip " + block_tag);

     encoded = TE_U8.encode(block);

     decoded = TD_U8.decode(encoded);

     assert_string_equals(block, decoded, "UTF-8 round trip " + block_tag);

     // test TextEncoder(UTF-8) against the older idiom

     exp_encoded = encode_utf8(block);

     assert_array_equals(encoded, exp_encoded,

                         "UTF-8 reference encoding " + block_tag);

     exp_decoded = decode_utf8(exp_encoded);

     assert_string_equals(decoded, exp_decoded,

                          "UTF-8 reference decoding " + block_tag);

   }

 }

 function test_utf_samples () {

   // z, cent, CJK water, G-Clef, Private-use character

   var sample = "z\xA2\u6C34\uD834\uDD1E\uDBFF\uDFFD";

   var cases = [

     { encoding: "utf-8",

       expected: [0x7A, 0xC2, 0xA2, 0xE6, 0xB0, 0xB4, 0xF0, 0x9D, 0x84, 0x9E, 0xF4, 0x8F, 0xBF, 0xBD] },

     { encoding: "utf-16le",

       expected: [0x7A, 0x00, 0xA2, 0x00, 0x34, 0x6C, 0x34, 0xD8, 0x1E, 0xDD, 0xFF, 0xDB, 0xFD, 0xDF] },

     { encoding: "utf-16",

       expected: [0x7A, 0x00, 0xA2, 0x00, 0x34, 0x6C, 0x34, 0xD8, 0x1E, 0xDD, 0xFF, 0xDB, 0xFD, 0xDF] },

     { encoding: "utf-16be",

       expected: [0x00, 0x7A, 0x00, 0xA2, 0x6C, 0x34, 0xD8, 0x34, 0xDD, 0x1E, 0xDB, 0xFF, 0xDF, 0xFD] }

   ];

   cases.forEach(

     function(t) {

       var encoded = new TextEncoder(t.encoding).encode(sample);

       assert_array_equals(encoded, t.expected,

                           "expected equal encodings - " + t.encoding);

       var decoded = new TextDecoder(t.encoding)

                         .decode(new Uint8Array(t.expected));

       assert_equals(decoded, sample,

                     "expected equal decodings - " + t.encoding);

     });

 }

 test(test_utf_samples,

      "UTF-8, UTF-16LE, UTF-16BE - Encode/Decode - reference sample");

 test(test_utf_roundtrip,

      "UTF-8, UTF-16LE, UTF-16BE - Encode/Decode - full roundtrip and "+

      "agreement with encode/decodeURIComponent");