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doc: add documentation for invalid byte sequences #28249

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doc: add documentation for invalid byte sequences #28249

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doc: add documentation for invalid byte sequences
rexagod Jun 16, 2019
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Update buffer.md
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Expand Up @@ -197,6 +197,38 @@ the WHATWG specification it is possible that the server actually returned
`'win-1252'`-encoded data, and using `'latin1'` encoding may incorrectly decode
the characters.

### Evaluating legal code points for '`utf-8'` encoding

Byte sequences that do not have corresponding UTF-16 encodings and non-legal
Unicode values, along with their UTF-8 counterparts must be treated as
invalid byte sequences.

For cases regarding operations other than employing backward compatibility
for 7-bit (and [extended 8-bit]((https://en.wikipedia.org/wiki/UTF-8#Description))
in rare cases) `'ascii'` data, and the valid [`UTF-8` code units](https://en.wikipedia.org/wiki/UTF-8#Codepage_layout),
it should be noted that the replacement character (`�`) is returned,
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and *no exception will be thrown*.
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It should also be noted that a `U+FFFD` replacement value
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(representing the aforementioned replacement character) will be returned
in case of decoding errors (invalid unicode scalar values).
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To be honest, I don’t understand most of the text or its relevance here… the text basically says that invalid UTF-8 byte sequences will be decoded into U+FFFD replacement characters and that no error will be thrown in those cases, right?

How do UTF-16 and ASCII relate to that? What does “non-legal Unicode value” mean? (I would guess that this refers to characters that would be beyond U+10FFFF – if that’s correct, can you clarify that in the text?)


```js
// Assuming an invalid byte sequence
const buf = Buffer.from([237, 166, 164]);

const buf_str = buf.toString('utf-8');

console.log(buf_str);
// Prints: '�'

console.log(buf.byteLength(buf_str));
// Prints: 3

console.log(buf.codePointAt(0).toString(16));
// Prints: 'fffd'
```

## Buffers and TypedArray
<!-- YAML
changes:
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