1 files changed, 47 insertions, 0 deletions
diff --git a/js/src/tests/test262/built-ins/String/prototype/isWellFormed/returns-boolean.js b/js/src/tests/test262/built-ins/String/prototype/isWellFormed/returns-boolean.js
new file mode 100644
index 0000000000..1109670e53
--- /dev/null
+++ b/js/src/tests/test262/built-ins/String/prototype/isWellFormed/returns-boolean.js
@@ -0,0 +1,47 @@
+// |reftest| shell-option(--enable-well-formed-unicode-strings) skip-if(!String.prototype.isWellFormed||!xulRuntime.shell) -- String.prototype.isWellFormed 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.iswellformed
+description: >
+  The method should return a boolean.
+info: |
+  String.prototype.isWellFormed ( )
+
+  1. Let O be ? RequireObjectCoercible(this value).
+  2. Let S be ? ToString(O).
+  3. Return IsStringWellFormedUnicode(S).
+
+features: [String.prototype.isWellFormed]
+---*/
+assert.sameValue(typeof String.prototype.isWellFormed, 'function');
+
+var leadingPoo = '\uD83D';
+var trailingPoo = '\uDCA9';
+var wholePoo = leadingPoo + trailingPoo;
+
+assert.sameValue(
+  ('a' + leadingPoo + 'c' + leadingPoo + 'e').isWellFormed(),
+  false,
+  'lone leading surrogates are not well-formed'
+);
+assert.sameValue(
+  ('a' + trailingPoo + 'c' + trailingPoo + 'e').isWellFormed(),
+  false,
+  'lone trailing surrogates are not well-formed'
+);
+assert.sameValue(
+  ('a' + trailingPoo + leadingPoo + 'd').isWellFormed(),
+  false,
+  'a wrong-ordered surrogate pair is not well-formed'
+)
+
+assert.sameValue('a💩c'.isWellFormed(), true, 'a surrogate pair using a literal code point is well-formed');
+assert.sameValue('a\uD83D\uDCA9c'.isWellFormed(), true, 'a surrogate pair formed by escape sequences is well-formed');
+assert.sameValue(('a' + leadingPoo + trailingPoo + 'd').isWellFormed(), true, 'a surrogate pair formed by concatenation is well-formed');
+assert.sameValue(wholePoo.slice(0, 1).isWellFormed(), false, 'a surrogate pair sliced to the leading surrogate is not well-formed');
+assert.sameValue(wholePoo.slice(1).isWellFormed(), false, 'a surrogate pair sliced to the trailing surrogate is not well-formed');
+assert.sameValue('abc'.isWellFormed(), true, 'a latin-1 string is well-formed');
+assert.sameValue('a\u25A8c'.isWellFormed(), true, 'a non-ASCII character is well-formed');
+
+reportCompare(0, 0);