es String 內(nèi)部實(shí)現(xiàn)邏輯標(biāo)準(zhǔn)

Jingbin_ 發(fā)布于2019-08-22 14:53 / 3115人閱讀

摘要：返回值類型是根據(jù)傳入的值是否為空字符串，選擇性調(diào)用方法代碼是一個(gè)抽象的操作，將傳入的參數(shù)值轉(zhuǎn)換為類型的值，這一過(guò)程是參照一個(gè)規(guī)則進(jìn)行，規(guī)則如下表傳入?yún)?shù)的類型返回的結(jié)果返回參數(shù)值參考下面詳細(xì)闡述調(diào)用方法，返回詳細(xì)闡述類型包括特殊的數(shù)值，還包

String(value)

返回值類型是 string value

根據(jù)傳入的值是否為空字符串，選擇性調(diào)用 ToString(value) 方法

代碼

function String(value) {
    return value === "" ? "" : ToString(value)
}

ToString(value)

ToString 是一個(gè)抽象的操作，將傳入的參數(shù)值轉(zhuǎn)換為 String 類型的值，這一過(guò)程是參照一個(gè)規(guī)則進(jìn)行，規(guī)則如下表

傳入?yún)?shù)的類型	返回的結(jié)果
Undefined	"undefined"
Null	"null"
Boolean	true => "true" false => "false"
String	返回參數(shù)值
Number	參考下面詳細(xì)闡述
Object	1. 調(diào)用 ToPrimitive 方法，var primValue = ToPrimitive(input argument hintString); 2. 返回 ToString(primValue)

詳細(xì)闡述 Number 類型

Number 包括 NaN 特殊的數(shù)值，還包括 Infinity 無(wú)限大和無(wú)限小的數(shù)值，包括能表示能存儲(chǔ)下的正常浮點(diǎn)數(shù)

以下 m 代替?zhèn)魅氲?Number 類型的參數(shù)的值

當(dāng) m 為 NaN，返回 String 形的 "NaN"

當(dāng) m 為 +0 或者 -0，返回 String 形的 "0"

當(dāng) m < 0, 返回 -ToString(-m)，意思是首先提取出 - 符號(hào)，再對(duì) -m 調(diào)用 ToString 方法

當(dāng) m 為 infinity，返回 String 形的 "Infinity"

以下為重點(diǎn)，當(dāng) m > 0 且不是 infinity 時(shí)的內(nèi)部抽象操作

Otherwise, let n, k, and s be integers such that k ≥ 1, 10k?1 ≤ s < 10k, the Number value for s × 10n?k is m, and k is as small as possible. Note that k is the number of digits in the decimal representation of s, that s is not divisible by 10, and that the least significant digit of s is not necessarily uniquely determined by these criteria.If k ≤ n ≤ 21, return the String consisting of the k digits of the
decimal representation of s (in order, with no leading zeroes),
followed by n?k occurrences of the character ‘0’.
If 0 < n ≤ 21, return the String consisting of the most significant n
digits of the decimal representation of s, followed by a decimal point
‘.’, followed by the remaining k?n digits of the decimal
representation of s.
If ?6 < n ≤ 0, return the String consisting of the character ‘0’,
followed by a decimal point ‘.’, followed by ?n occurrences of the
character ‘0’, followed by the k digits of the decimal representation
of s.
Otherwise, if k = 1, return the String consisting of the single digit
of s, followed by lowercase character ‘e’, followed by a plus sign ‘+’
or minus sign ‘?’ according to whether n?1 is positive or negative,
followed by the decimal representation of the integer abs(n?1) (with
no leading zeros).
Return the String consisting of the most significant digit of the
decimal representation of s, followed by a decimal point ‘.’, followed
by the remaining k?1 digits of the decimal representation of s,
followed by the lowercase character ‘e’, followed by a plus sign ‘+’
or minus sign ‘?’ according to whether n?1 is positive or negative,
followed by the decimal representation of the integer abs(n?1) (with
no leading zeros).