Emoji 正则匹配

Strings are represented fundamentally as sequences of UTF-16 code units. In UTF-16 encoding, every code unit is exact 16 bits long. This means there are a maximum of 2^16, or 65536 possible characters representable as single UTF-16 code units.

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However, the entire Unicode character set is much, much bigger than 65536. The extra characters are stored in UTF-16 as surrogate pairs, which are pairs of 16-bit code units that represent a single character.To avoid ambiguity, the two parts of the pair must be between 0xD800 and 0xDFFF, and these code units are not used to encode single-code-unit characters. (More precisely, leading surrogates, also called high-surrogate code units, have values between 0xD800 and 0xDBFF, inclusive, while trailing surrogates, also called low-surrogate code units, have values between 0xDC00 and 0xDFFF, inclusive.) Each Unicode character, comprised of one or two UTF-16 code units, is also called a Unicode code point. Each Unicode code point can be written in a string with \u{xxxxxx} where xxxxxx represents 1–6 hex digits.

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String.prototype.charAt()

The charAt() method of String values returns a new string consisting of the single UTF-16 code unit at the given index.

charAt() 返回字符串对应下标的单个 UTF-16 编码单元。

'🉐'.charAt(0) // '\uD83C'
'🉐'.charAt(1) // '\uDE50'

'a'.charAt(0) // 'a'
'a'.charAt(1) // ''

'apple'.charAt(0) // 'a'
'apple'.charAt(1) // 'p'

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String.property.charCodeAt()

The charCodeAt() method of String values returns an integer between 0 and 65535 representing the UTF-16 code unit at the given index.

charCodeAt() 返回字符一个 0-65535 之间的 数字(integer) , 是字符串对应下标的 UTF-16 编码单元对应的数值。

'🉐'.charCodeAt(0) // 55356
'🉐'.charCodeAt(1) // 56912

'a'.charCodeAt(0) // 97
'a'.charCodeAt(1) // NaN

'apple'.charCodeAt(0) // 97
'apple'.charCodeAt(1) // 112

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String.fromCharCode()

The String.fromCharCode() static method returns a string created from the specified sequence of UTF-16 code units.

fromCharCode() 可以接受多个 0-65535 之间的数字，返回这些 char code 组成的字符串。

fromCharCode() 和 charCodeAt() 是对应的。

'🉐'.charCodeAt(0) // 55356
'🉐'.charCodeAt(1) // 56912
String.fromCharCode(55356, 56912) // '🉐'

'apple'.charCodeAt(0) // 97
'apple'.charCodeAt(1) // 112
'apple'.charCodeAt(2) // 112
'apple'.charCodeAt(3) // 108
'apple'.charCodeAt(4) // 101
String.fromCharCode(97, 112, 112, 108, 101) // 'apple'

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String.prototype.codePointAt()

The codePointAt() method of String values returns a non-negative integer that is the Unicode code point value of the character starting at the given index.

Note that the index is still based on UTF-16 code units, not Unicode code points.

codePointAt() 返回的是一个数字，是字符串对应下标的对应的 Unicode code point (不局限在 0-65535, 而是 0-1114111 (0x10FFFF))。

'🉐'.charCodeAt(0) // 55356
'🉐'.charCodeAt(1) // 56912

'🉐'.codePointAt(0) // 127568
// 需要注意的是，下标是基于 UTF-16 计算的，🉐 是由两个 UTF-16 组成的
// 对于下标 0，可以找到一个代理对，对应 Unicode code point，所以返回了 127568
// 对于下标 1，由于只有低代理位，无法组成代理对，就返回低代理位对应的 Unicode Code Point
'🉐'.codePointAt(1) // 56912

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String.fromCodePoint()

The String.fromCodePoint() static method returns a string created from the specified sequence of code points.

fromCodePoint() 可以接受多个 codePoint 数字，返回对应的字符串。

fromCharCode() 的每个参数只能是 0-65535 范围的数字，而 fromCodePoint() 则可以输入 0-1114111 范围的数字。

// fromCodePoint
String.fromCodePoint(127568) // '🉐'
String.fromCodePoint(55356, 56912) // '🉐'

// fromCharCode
// fromCharCode 只能处理 0-65535 之间的数字，对于超过 65535 的数字，则截断到最后的 16 位数字
// 等同于 String.fromCharCode(62032)
// Number(127568).toString(16) -> '1f250' -> 截取最后的 16 位，即 f250
// Number.parseInt('f250', 16) -> 62032
String.fromCharCode(127568) // ''
String.fromCharCode(55356, 56912) // '🉐'

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