为什么String（null）有效？ [英] Why does String(null) work?

问题描述

null 和 undefined 没有 toString 或 valueOf 方法。 Afaik使用 String 调用其参数的 toString 方法（例如 String（{}） => [object Object] ）。

null and undefined don't have a toString or valueOf method. Afaik using String calls the toString method of its parameter (e.g. String({}) => [object Object]).

为什么 String（null）或 String（undefined 然后工作？它不隐式地执行 Object.prototype.toString.call（null）。因为它的计算结果为 [object Null] 。

Why do String(null) or String(undefined work then? It doesn't implicitly do Object.prototype.toString.call(null). because that evaluates to [object Null].

：来自ECMA-262 / 5th版规格（第48页）。这不会增加澄清，我会说：

[edit]: from the spec ECMA-262/5th edition (page 48). This doesn't add to clarification, I'd say:

/*
Table 13 — ToString Conversions  
-------------------------------------------------------------------------
Argument Type  | Result  
-------------------------------------------------------------------------
Undefined      | "undefined"
Null           | "null"  
Boolean        | If the argument is true, then the result is "true".
...            | ...
*/

推荐答案

在查看我以前的答案后，它似乎对我之前的答案进行了彻底的改革是必要的。我正在将它复杂化，就像简短的回答一样r是这些是标准规定的特殊情况。

After reviewing my previous answer, it seems a complete overhaul of my previous answer is necessary. I was way over complicating it, as the short answer is that these are standards-specified special cases.

规范 字符串（）（字符串用作函数）：

15.5.1.1字符串（[value]）

15.5.1.1 String ( [ value ] )

返回由ToString（value）计算的String值（不是String对象）。如果未提供value，则返回空的
String。

Returns a String value (not a String object) computed by ToString(value). If value is not supplied, the empty String "" is returned.

ToString 函数（内部存在，不存在于userland中）已定义如下（9.8）：

The ToString function (that exists internally, not in userland) is defined as follows (9.8):

抽象操作ToString根据表13将其参数转换为String类型的值

"The abstract operation ToString converts its argument to a value of type String according to Table 13"

Argument Type | Result
Null | "null"
Undefined | "undefined"

这意味着 String（null）和字符串（未定义）进入这个特殊的类型表，只返回值为null的字符串值和undefined。

This means that String(null) and String(undefined) go into this special table of types and just return the string values valued "null" and "undefined".

user-land伪实现看起来像这样：

A user-land pseudo-implementation looks something like this:

function MyString(val) {
    if (arguments.length === 0) {
        return "";
    } else if (typeof val === "undefined") {
        return "undefined";
    } else if (val === null) {
        return "null";
    } else if (typeof val === "boolean") {
        return val ? "true" : "false";
    } else if (typeof val === "number") {
        // super complex rules
    } else if (typeof val === "string") {
        return val;
    } else {
        // return MyString(ToPrimitive(val, prefer string))
    }
}

（注意，这个例子忽略了构造函数的情况（ new MyString（））并且它使用了用户区概念而不是比发动机陆地。）

(Note that this example ignores the constructor case (new MyString()) and that it uses user-land concepts rather than engine-land.)

我有点匆匆忙忙地找到了一个示例实现（V8具体）：

I got a bit carried away and found an example implementation (V8 to be specific):

string.js：

// Set the String function and constructor.
%SetCode($String, function(x) {
  var value = %_ArgumentsLength() == 0 ? '' : TO_STRING_INLINE(x);
  if (%_IsConstructCall()) {
    %_SetValueOf(this, value);
  } else {
    return value;
  }
});

macros.py:

macro TO_STRING_INLINE(arg) = (IS_STRING(%IS_VAR(arg)) ? arg : NonStringToString(arg));

runtime.js：

function NonStringToString(x) {
  if (IS_NUMBER(x)) return %_NumberToString(x);
  if (IS_BOOLEAN(x)) return x ? 'true' : 'false';
  if (IS_UNDEFINED(x)) return 'undefined';
  return (IS_NULL(x)) ? 'null' : %ToString(%DefaultString(x));
}

NonStringToString（基本上是我感兴趣的），幸运的是在伪JS-土地。正如您所看到的，确实存在null / true / false / undefined的特殊情况。

The NonStringToString (which is essentially what is of interest), is luckily defined in psuedo-JS-land. As you can see, there is indeed a special case for null/true/false/undefined.

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