Javascript 为什么 String(null) 有效？
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/10362114/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
Why does String(null) work?
提问by KooiInc
nulland undefineddon't have a toStringor valueOfmethod. Afaik using Stringcalls the toStringmethod of its parameter (e.g. String({})=> [object Object]).
null并且undefined没有toStringorvalueOf方法。Afaik usingString调用toString其参数的方法（例如String({})=> [object Object]）。
Why do String(null)or String(undefinedwork then? It doesn't implicitly do Object.prototype.toString.call(null). because that evaluates to [object Null].
那为什么要工作String(null)或String(undefined工作呢？它不会隐含地做Object.prototype.toString.call(null)。因为它评估为[object Null].
[edit]: from the spec ECMA-262/5th edition (page 48). This doesn't add to clarification, I'd say:
[编辑]：来自规范 ECMA-262/5th edition（第 48 页）。这并没有增加澄清，我会说：
/*
Table 13 — ToString Conversions
-------------------------------------------------------------------------
Argument Type | Result
-------------------------------------------------------------------------
Undefined | "undefined"
Null | "null"
Boolean | If the argument is true, then the result is "true".
... | ...
*/
回答by Corbin
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.
在查看了我之前的答案之后，似乎有必要对我之前的答案进行彻底检查。我把它复杂化了，因为简短的回答是这些是标准指定的特殊情况。
The specificationfor String()(Stringused as a function):
的规范为String()（String用作函数）：
15.5.1.1 String ( [ value ] )
Returns a String value (not a String object) computed by ToString(value). If value is not supplied, the empty String "" is returned.
15.5.1.1 字符串（[值]）
返回由 ToString(value) 计算的 String 值（不是 String 对象）。如果未提供 value，则返回空字符串 ""。
The ToStringfunction (that exists internally, not in userland) is defined as follows (9.8):
的ToString功能（即在内部存在，而不是在用户级）被定义如下（9.8）：
"The abstract operation ToString converts its argument to a value of type String according to Table 13"
“抽象操作 ToString 根据表 13 将其参数转换为 String 类型的值”
Argument Type | Result
Null | "null"
Undefined | "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".
这意味着String(null)andString(undefined)进入这个特殊的类型表并只返回值为"null"and的字符串值"undefined"。
A user-land pseudo-implementation looks something like this:
用户态伪实现如下所示：
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))
}
}
(Note that this example ignores the constructor case (new MyString()) and that it uses user-land concepts rather than engine-land.)
（请注意，此示例忽略了构造函数案例 ( new MyString()) 并且它使用了用户空间概念而不是引擎空间。）
I got a bit carried away and found an example implementation (V8 to be specific):
我有点忘乎所以，找到了一个示例实现（具体来说是 V8）：
string.js:
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:
macros.py:
macro TO_STRING_INLINE(arg) = (IS_STRING(%IS_VAR(arg)) ? arg : NonStringToString(arg));
runtime.js:
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));
}
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.
NonStringToString（本质上是感兴趣的），幸运的是在 psuedo-JS-land 中定义。如您所见，确实存在 null/true/false/undefined 的特殊情况。
回答by Thilo
There is probably just some extra checks and handling for special cases like nulland undefined.
有可能只是一些额外的检查和处理的特殊情况下，像null和undefined。
It's possible to use String as a "safer" toString alternative, as although it still normally calls the underlying toString, it also works for null and undefined.
可以将 String 用作“更安全”的 toString 替代方案，尽管它通常仍会调用底层 toString，但它也适用于 null 和 undefined。
回答by Dan D.
You might be interested in seeing the Annotated ES5(which is much more readable than the ECMAScript 5 PDF) which states that: new String([ value ])http://es5.github.com/#x15.5.2.1calls [ToString]http://es5.github.com/#x9.8(there is a table of the special convertion cases) to convert the value passed to it to a string.
你可能有兴趣在看到注释ES5（这是比的ECMAScript 5 PDF更可读），其中指出：new String([ value ])http://es5.github.com/#x15.5.2.1调用[ToString]HTTP：//es5.github。 com/#x9.8（有特殊转换情况的表格）将传递给它的值转换为字符串。
回答by robrich
String(null)creates a string object and passes it a default value of null.
String(null)创建一个字符串对象并将默认值传递给它。
