Merge pull request #290 from afilahkle/master

adding missing article.

Author: afilahkle

1-js/99-js-misc/06-unicode/article.md

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+
+# Unicode, String internals
+
+```warn header="Advanced knowledge"
+The section goes deeper into string internals. This knowledge will be useful for you if you plan to deal with emoji, rare mathematical or hieroglyphic characters, or other rare symbols.
+```
+
+As we already know, JavaScript strings are based on [Unicode](https://en.wikipedia.org/wiki/Unicode): each character is represented by a byte sequence of 1-4 bytes.
+
+JavaScript allows us to insert a character into a string by specifying its hexadecimal Unicode code with one of these three notations:
+
+- `\xXX`
+
+    `XX` must be two hexadecimal digits with a value between `00` and `FF`, then `\xXX` is the character whose Unicode code is `XX`.
+
+    Because the `\xXX` notation supports only two hexadecimal digits, it can be used only for the first 256 Unicode characters.
+
+    These first 256 characters include the Latin alphabet, most basic syntax characters, and some others. For example, `"\x7A"` is the same as `"z"` (Unicode `U+007A`).
+
+    ```js run
+    alert( "\x7A" ); // z
+    alert( "\xA9" ); // ©, the copyright symbol
+    ```
+
+- `\uXXXX`
+    `XXXX` must be exactly 4 hex digits with the value between `0000` and `FFFF`, then `\uXXXX` is the character whose Unicode code is `XXXX`.
+
+    Characters with Unicode values greater than `U+FFFF` can also be represented with this notation, but in this case, we will need to use a so called surrogate pair (we will talk about surrogate pairs later in this chapter).
+
+    ```js run
+    alert( "\u00A9" ); // ©, the same as \xA9, using the 4-digit hex notation
+    alert( "\u044F" ); // я, the Cyrillic alphabet letter
+    alert( "\u2191" ); // ↑, the arrow up symbol
+    ```
+
+- `\u{X…XXXXXX}`
+
+    `X…XXXXXX` must be a hexadecimal value of 1 to 6 bytes between `0` and `10FFFF` (the highest code point defined by Unicode). This notation allows us to easily represent all existing Unicode characters.
+
+    ```js run
+    alert( "\u{20331}" ); // 佫, a rare Chinese character (long Unicode)
+    alert( "\u{1F60D}" ); // 😍, a smiling face symbol (another long Unicode)
+    ```
+
+## Surrogate pairs
+
+All frequently used characters have 2-byte codes (4 hex digits). Letters in most European languages, numbers, and the basic unified CJK ideographic sets (CJK -- from Chinese, Japanese, and Korean writing systems), have a 2-byte representation.
+
+Initially, JavaScript was based on UTF-16 encoding that only allowed 2 bytes per character. But 2 bytes only allow 65536 combinations and that's not enough for every possible symbol of Unicode.
+
+So rare symbols that require more than 2 bytes are encoded with a pair of 2-byte characters called "a surrogate pair".
+
+As a side effect, the length of such symbols is `2`:
+
+```js run
+alert( '𝒳'.length ); // 2, MATHEMATICAL SCRIPT CAPITAL X
+alert( '😂'.length ); // 2, FACE WITH TEARS OF JOY
+alert( '𩷶'.length ); // 2, a rare Chinese character
+```
+
+That's because surrogate pairs did not exist at the time when JavaScript was created, and thus are not correctly processed by the language!
+
+We actually have a single symbol in each of the strings above, but the `length` property shows a length of `2`.
+
+Getting a symbol can also be tricky, because most language features treat surrogate pairs as two characters.
+
+For example, here we can see two odd characters in the output:
+
+```js run
+alert( '𝒳'[0] ); // shows strange symbols...
+alert( '𝒳'[1] ); // ...pieces of the surrogate pair
+```
+
+Pieces of a surrogate pair have no meaning without each other. So the alerts in the example above actually display garbage.
+
+Technically, surrogate pairs are also detectable by their codes: if a character has the code in the interval of `0xd800..0xdbff`, then it is the first part of the surrogate pair. The next character (second part) must have the code in interval `0xdc00..0xdfff`. These intervals are reserved exclusively for surrogate pairs by the standard.
+
+So the methods [String.fromCodePoint](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/fromCodePoint) and [str.codePointAt](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/codePointAt) were added in JavaScript to deal with surrogate pairs.
+
+They are essentially the same as [String.fromCharCode](mdn:js/String/fromCharCode) and [str.charCodeAt](mdn:js/String/charCodeAt), but they treat surrogate pairs correctly.
+
+One can see the difference here:
+
+```js run
+// charCodeAt is not surrogate-pair aware, so it gives codes for the 1st part of 𝒳:
+
+alert( '𝒳'.charCodeAt(0).toString(16) ); // d835
+
+// codePointAt is surrogate-pair aware
+alert( '𝒳'.codePointAt(0).toString(16) ); // 1d4b3, reads both parts of the surrogate pair
+```
+
+That said, if we take from position 1 (and that's rather incorrect here), then they both return only the 2nd part of the pair:
+
+```js run
+alert( '𝒳'.charCodeAt(1).toString(16) ); // dcb3
+alert( '𝒳'.codePointAt(1).toString(16) ); // dcb3
+// meaningless 2nd half of the pair
+```
+
+You will find more ways to deal with surrogate pairs later in the chapter <info:iterable>. There are probably special libraries for that too, but nothing famous enough to suggest here.
+
+````warn header="Takeaway: splitting strings at an arbitrary point is dangerous"
+We can't just split a string at an arbitrary position, e.g. take `str.slice(0, 4)` and expect it to be a valid string, e.g.:
+
+```js run
+alert( 'hi 😂'.slice(0, 4) ); //  hi [?]
+```
+
+Here we can see a garbage character (first half of the smile surrogate pair) in the output.
+
+Just be aware of it if you intend to reliably work with surrogate pairs. May not be a big problem, but at least you should understand what happens.
+````
+
+## Diacritical marks and normalization
+
+In many languages, there are symbols that are composed of the base character with a mark above/under it.
+
+For instance, the letter `a` can be the base character for these characters: `àáâäãåā`.
+
+Most common "composite" characters have their own code in the Unicode table. But not all of them, because there are too many possible combinations.
+
+To support arbitrary compositions, the Unicode standard allows us to use several Unicode characters: the base character followed by one or many "mark" characters that "decorate" it.
+
+For instance, if we have `S` followed by the special "dot above" character (code `\u0307`), it is shown as Ṡ.
+
+```js run
+alert( 'S\u0307' ); // Ṡ
+```
+
+If we need an additional mark above the letter (or below it) -- no problem, just add the necessary mark character.
+
+For instance, if we append a character "dot below" (code `\u0323`), then we'll have "S with dots above and below": `Ṩ`.
+
+For example:
+
+```js run
+alert( 'S\u0307\u0323' ); // Ṩ
+```
+
+This provides great flexibility, but also an interesting problem: two characters may visually look the same, but be represented with different Unicode compositions.
+
+For instance:
+
+```js run
+let s1 = 'S\u0307\u0323'; // Ṩ, S + dot above + dot below
+let s2 = 'S\u0323\u0307'; // Ṩ, S + dot below + dot above
+
+alert( `s1: ${s1}, s2: ${s2}` );
+
+alert( s1 == s2 ); // false though the characters look identical (?!)
+```
+
+To solve this, there exists a "Unicode normalization" algorithm that brings each string to the single "normal" form.
+
+It is implemented by [str.normalize()](mdn:js/String/normalize).
+
+```js run
+alert( "S\u0307\u0323".normalize() == "S\u0323\u0307".normalize() ); // true
+```
+
+It's funny that in our situation `normalize()` actually brings together a sequence of 3 characters to one: `\u1e68` (S with two dots).
+
+```js run
+alert( "S\u0307\u0323".normalize().length ); // 1
+
+alert( "S\u0307\u0323".normalize() == "\u1e68" ); // true
+```
+
+In reality, this is not always the case. The reason is that the symbol `Ṩ` is "common enough", so Unicode creators included it in the main table and gave it the code.
+
+If you want to learn more about normalization rules and variants -- they are described in the appendix of the Unicode standard: [Unicode Normalization Forms](https://www.unicode.org/reports/tr15/), but for most practical purposes the information from this section is enough.