1 files changed, 58 insertions, 0 deletions
diff --git a/js/src/tests/test262/built-ins/String/prototype/toWellFormed/returns-well-formed-string.js b/js/src/tests/test262/built-ins/String/prototype/toWellFormed/returns-well-formed-string.js
new file mode 100644
index 0000000000..df77f0c598
--- /dev/null
+++ b/js/src/tests/test262/built-ins/String/prototype/toWellFormed/returns-well-formed-string.js
@@ -0,0 +1,58 @@
+// |reftest| shell-option(--enable-well-formed-unicode-strings) skip-if(!String.prototype.toWellFormed||!xulRuntime.shell) -- String.prototype.toWellFormed is not enabled unconditionally, requires shell-options
+// Copyright (C) 2022 Jordan Harband. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-string.prototype.towellformed
+description: >
+  The method should return a well-formed string.
+info: |
+  String.prototype.toWellFormed ( )
+
+  1. Let O be ? RequireObjectCoercible(this value).
+  2. Let S be ? ToString(O).
+  3. Let strLen be the length of S.
+  4. Let k be 0.
+  5. Let result be the empty String.
+  6. Repeat, while k < strLen,
+    a. Let cp be CodePointAt(S, k).
+    b. If cp.[[IsUnpairedSurrogate]] is true, then
+      i. Set result to the string-concatenation of result and 0xFFFD (REPLACEMENT CHARACTER).
+    c. Else,
+      i. Set result to the string-concatenation of result and UTF16EncodeCodePoint(cp.[[CodePoint]]).
+    d. Set k to k + cp.[[CodeUnitCount]].
+  7. Return result.
+
+features: [String.prototype.toWellFormed]
+---*/
+assert.sameValue(typeof String.prototype.toWellFormed, 'function');
+
+var replacementChar = '\uFFFD';
+var leadingPoo = '\uD83D';
+var trailingPoo = '\uDCA9';
+var wholePoo = leadingPoo + trailingPoo;
+
+assert.sameValue(
+  ('a' + leadingPoo + 'c' + leadingPoo + 'e').toWellFormed(),
+  'a' + replacementChar + 'c' + replacementChar + 'e',
+  'lone leading surrogates are replaced with the expected replacement character'
+);
+assert.sameValue(
+  ('a' + trailingPoo + 'c' + trailingPoo + 'e').toWellFormed(),
+  'a' + replacementChar + 'c' + replacementChar + 'e',
+  'lone trailing surrogates are replaced with the expected replacement character'
+);
+assert.sameValue(
+  ('a' + trailingPoo + leadingPoo + 'd').toWellFormed(),
+  'a' + replacementChar + replacementChar + 'd',
+  'a wrong-ordered surrogate pair is replaced with two replacement characters'
+)
+
+assert.sameValue('a💩c'.toWellFormed(), 'a💩c', 'a surrogate pair using a literal code point is already well-formed');
+assert.sameValue('a\uD83D\uDCA9c'.toWellFormed(), 'a\uD83D\uDCA9c', 'a surrogate pair formed by escape sequences is already well-formed');
+assert.sameValue(('a' + leadingPoo + trailingPoo + 'd').toWellFormed(), 'a' + wholePoo + 'd', 'a surrogate pair formed by concatenation is already well-formed');
+assert.sameValue(wholePoo.slice(0, 1).toWellFormed(), replacementChar, 'a surrogate pair sliced to the leading surrogate is replaced with the expected replacement character');
+assert.sameValue(wholePoo.slice(1).toWellFormed(), replacementChar, 'a surrogate pair sliced to the trailing surrogate is replaced with the expected replacement character');
+assert.sameValue('abc'.toWellFormed(), 'abc', 'a latin-1 string is already well-formed');
+assert.sameValue('a\u25A8c'.toWellFormed(), 'a\u25A8c', 'a string with a non-ASCII character is already well-formed');
+
+reportCompare(0, 0);