gecko-dev/testing/web-platform/tests/encoding/textencoder-utf16-surrogates.html

<!DOCTYPE html>
<title>Encoding API: USVString surrogate handling when encoding</title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script>

var bad = [
    {
        input: '\uD800',
        expected: '\uFFFD',
        name: 'lone surrogate lead'
    },
    {
        input: '\uDC00',
        expected: '\uFFFD',
        name: 'lone surrogate trail'
    },
    {
        input: '\uD800\u0000',
        expected: '\uFFFD\u0000',
        name: 'unmatched surrogate lead'
    },
    {
        input: '\uDC00\u0000',
        expected: '\uFFFD\u0000',
        name: 'unmatched surrogate trail'
    },
    {
        input: '\uDC00\uD800',
        expected: '\uFFFD\uFFFD',
        name: 'swapped surrogate pair'
    },
    {
        input: '\uD834\uDD1E',
        expected: '\uD834\uDD1E',
        name: 'properly encoded MUSICAL SYMBOL G CLEF (U+1D11E)'
    }
];

var encoding = 'utf-8';

bad.forEach(function(t) {
    test(function() {
        var encoded = new TextEncoder(encoding).encode(t.input);
        var decoded = new TextDecoder(encoding).decode(encoded);
        assert_equals(decoded, t.expected);
    }, 'USVString handling: ' + t.name);
});

test(function() {
    assert_equals(new TextEncoder(encoding).encode().length, 0, 'Should default to empty string');
}, 'USVString default');

</script>