JavaScriptのUnicode

JavaScriptでUnicodeを操作する方法、絵文字が何でできているか、ES6の改善、JSでUnicodeを処理する際のいくつかの落とし穴について学びます。

ソースファイルのUnicodeエンコーディング

特に指定がない限り、ブラウザはプログラムのソースコードがローカル文字セットで記述されていると想定します。これは国によって異なり、予期しない問題が発生する可能性があります。このため、JavaScriptドキュメントの文字セットを設定することが重要です。

別のエンコーディング、特にWeb上で最も一般的なファイルエンコーディングであるUTF-8をどのように指定しますか？

ファイルにが含まれている場合BOM文字、それはエンコーディングを決定することを優先します。オンラインでさまざまな意見を読むことができます。UTF-8のBOMは推奨されないと言う人もいれば、追加しない編集者もいます。

これは何ですかUnicode標準は言う：

…UTF-8ではBOMの使用は必須でも推奨でもありませんが、UTF-8データがBOMを使用する他のエンコード形式から変換される場合、またはBOMがUTF-8署名として使用される場合に発生する可能性があります。

これはW3Cが言うことです：

HTML5では、ブラウザーはUTF-8 BOMを認識し、それを使用してページのエンコードを検出する必要があります。最近のバージョンの主要なブラウザーは、UTF-8エンコードされたページに使用すると期待どおりにBOMを処理します。 –https://www.w3.org/International/questions/qa-byte-order-mark

ファイルがHTTP（またはHTTPS）を使用してフェッチされる場合、Content-Typeヘッダーエンコーディングを指定できます：

Content-Type: application/javascript; charset=utf-8
If this is not set, the fallback is to check the charset attribute of the script tag:


<script src="./app.js" charset="utf-8">


If this is not set, the document charset meta tag is used:


...
<head>
  <meta charset="utf-8" />
</head>
...


The charset attribute in both cases is case insensitive (see the spec)


All this is defined in RFC 4329 “Scripting Media Types”.


Public libraries should generally avoid using characters outside the ASCII set in their code, to avoid it being loaded by users with an encoding that is different than their original one, and thus create issues.


How JavaScript uses Unicode internally


While a JavaScript source file can have any kind of encoding, JavaScript will then convert it internally to UTF-16 before executing it.


JavaScript strings are all UTF-16 sequences, as the ECMAScript standard says:




When a String contains actual textual data, each element is considered to be a single UTF-16 code unit.




Using Unicode in a string


A unicode sequence can be added inside any string using the format \uXXXX:


const s1 = '\u00E9' //é



A sequence can be created by combining two unicode sequences:


const s2 = '\u0065\u0301' //é



Notice that while both generate an accented e, they are two different strings, and s2 is considered to be 2 characters long:


s1.length //1
s2.length //2



And when you try to select that character in a text editor, you need to go through it 2 times, as the first time you press the arrow key to select it, it just selects half element.


You can write a string combining a unicode character with a plain char, as internally it’s actually the same thing:


const s3 = 'e\u0301' //é
s3.length === 2 //true
s2 === s3 //true
s1 !== s3 //true



Normalization


Unicode normalization is the process of removing ambiguities in how a character can be represented, to aid in comparing strings, for example.


Like in the example above:


const s1 = '\u00E9' //é
const s3 = 'e\u0301' //é
s1 !== s3



ES6/ES2015 introduced the normalize() method on the String prototype, so we can do:


s1.normalize() === s3.normalize() //true



Emojis


Emojis are fun, and they are Unicode characters, and as such they are perfectly valid to be used in strings:


const s4 = '🐶'



Emojis are part of the astral planes, outside of the first Basic Multilingual Plane (BMP), and since those points outside BMP cannot be represented in 16 bits, JavaScript needs to use a combination of 2 characters to represent them


The 🐶 symbol, which is U+1F436, is traditionally encoded as \uD83D\uDC36 (called surrogate pair). There is a formula to calculate this, but it’s a rather advanced topic.


Some emojis are also created by combining together other emojis. You can find those by looking at this list https://unicode.org/emoji/charts/full-emoji-list.html and notice the ones that have more than one item in the unicode symbol column.


👩‍❤️‍👩 is created combining 👩 (\uD83D\uDC69), ❤️‍ (\u200D\u2764\uFE0F\u200D) and another 👩 (\uD83D\uDC69) in a single string: \uD83D\uDC69\u200D\u2764\uFE0F\u200D\uD83D\uDC69


There is no way to make this emoji be counted as 1 character.


Get the proper length of a string


If you try to perform


'👩‍❤️‍👩'.length



You’ll get 8 in return, as length counts the single Unicode code points.


Also, iterating over it is kind of funny:


Iterating an emoji


And curiously, pasting this emoji in a password field it’s counted 8 times, possibly making it a valid password in some systems.


How to get the “real” length of a string containing unicode characters?


One easy way in ES6+ is to use the spread operator:


;[...'🐶'].length //1



You can also use the Punycode library by Mathias Bynens:


require('punycode').ucs2.decode('🐶').length //1



(Punycode is also great to convert Unicode to ASCII)


Note that emojis that are built by combining other emojis will still give a bad count:


require('punycode').ucs2.decode('👩‍❤️‍👩').length //6
[...'👩‍❤️‍👩'].length //6



If the string has combining marks however, this still will not give the right count. Check this Glitch https://glitch.com/edit/#!/node-unicode-ignore-marks-in-length as an example.


(you can generate your own weird text with marks here: https://lingojam.com/WeirdTextGenerator)


Length is not the only thing to pay attention. Also reversing a string is error prone if not handled correctly.


ES6 Unicode code point escapes


ES6/ES2015 introduced a way to represent Unicode points in the astral planes (any Unicode code point requiring more than 4 chars), by wrapping the code in graph parentheses:


'\u{XXXXX}'



The dog 🐶 symbol, which is U+1F436, can be represented as \u{1F436} instead of having to combine two unrelated Unicode code points, like we showed before: \uD83D\uDC36.


But length calculation still does not work correctly, because internally it’s converted to the surrogate pair shown above.


Encoding ASCII chars


The first 128 characters can be encoded using the special escaping character \x, which only accepts 2 characters:


'\x61' // a
'\x2A' // *



This will only work from \x00 to \xFF, which is the set of ASCII characters.

  



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