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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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doc: add documentation for invalid byte sequences 
added documentation on evaluating legal code points,
and the behavior that stems from it otherwise.

Fixes: #23280
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rexagod committed Jun 16, 2019
commit aa01448ab8b88dd45ce9221eef9d65bffd4f59ca
32 changes: 32 additions & 0 deletions doc/api/buffer.md
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Expand Up @@ -165,6 +165,38 @@ console.log(Buffer.from('fhqwhgads', 'utf16le'));
// Prints: <Buffer 66 00 68 00 71 00 77 00 68 00 67 00 61 00 64 00 73 00>
```

### Evaluating legal code points for '`utf-8'` encoding
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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,
and *no exception will be thrown*.

It should also be noted that a `U+FFFD` replacement value
(representing the aforementioned replacement character) will be returned
in case of decoding errors (invalid unicode scalar values).

```js
// assume 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'
```

The character encodings currently supported by Node.js include:

* `'ascii'` - For 7-bit ASCII data only. This encoding is fast and will strip
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