String(value)
- 返回值范例是 string value
- 依据传入的值是不是为空字符串，选择性挪用 ToString(value) 要领
代码
function String(value) {
return value === "" ? "" : ToString(value)
}ToString(value)
ToString 是一个笼统的操纵，将传入的参数值转换为 String 范例的值，这一历程是参照一个划定规矩举行，划定规矩如下表
| 传入参数的范例 | 返回的效果 |
|---|---|
| Undefined | “undefined” |
| Null | “null” |
| Boolean | true => “true” false => “false” |
| String | 返回 参数值 |
| Number | 参考下面细致论述 |
| Object | 1. 挪用 ToPrimitive 要领，var primValue = ToPrimitive(input argument hintString); 2. 返回 ToString(primValue) |
细致论述 Number 范例
Number 包含 NaN 特别的数值，还包含 Infinity 无穷大和无穷小的数值，包含能示意能存储下的一般浮点数
以下 m 替代传入的 Number 范例的参数的值
- 当 m 为 NaN，返回 String 形的 “NaN”
- 当 m 为 +0 或许 -0，返回 String 形的 “0”
- 当 m < 0, 返回 -ToString(-m)，意义是起首提掏出 – 标记，再对 -m 挪用 ToString 要领
- 当 m 为 infinity，返回 String 形的 “Infinity”
- 以下为重点，当 m > 0 且不是 infinity 时的内部笼统操纵
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).
浅显的明白就是 科学计数法示意
参考
http://es5.github.io/#x15.5.1.1
http://es5.github.io/#x9.8
