String（value）与value.toString（）之间的区别是什么 [英] What's the difference between String(value) vs value.toString()

问题描述

Javascript在类型和类型转换方面有很多技巧，所以我想知道这两种方法是否相同，或者是否存在一些使它们不同的极端情况？

Javascript has lot's of "tricks" around types and type conversions so I'm wondering if these 2 methods are the same or if there is some corner case that makes them different?

推荐答案

它们并不完全相同，实际上，作为函数调用的String构造函数（你的第一个例子），最后会调用传递的对象的 toString 方法，例如：

They are not completely the same, and actually, the String constructor called as a function (your first example), will at the end, call the toString method of the object passed, for example:

var o = { toString: function () { return "foo"; } };
String(o); // "foo"

另一方面，如果标识符引用 null 或 undefined ，你不能使用 toString 方法，它会给你a TypeError 异常：

On the other hand, if an identifier refers to null or undefined, you can't use the toString method, it will give you a TypeError exception:

var value = null;
String(null);     // "null"
value.toString(); // TypeError

String 构造函数被调用为函数大致相当于：

The String constructor called as a function would be roughly equivalent to:

value + '';

Object -to- Primitive <的类型转换规则/ em>详细描述了规范， [ [DefaultValue]] 内部操作。

The type conversion rules from Object-to-Primitive are detailed described on the specification, the [[DefaultValue]] internal operation.

简要概括，从 Object转换 - 到 - String ，采取以下步骤：

Briefly summarized, when converting from Object-to-String, the following steps are taken:

1. 如果可用，执行 toString 方法。
   
   
   
   
   • 如果结果是原语，则返回结果，否则转到步骤2.
   
   

1. If available, execute the toString method.
   • If the result is a primitive, return result, else go to Step 2.

• 如果结果是原语，则返回结果，否则转到步骤3.

• If the result is a primitive, return result, else go to Step 3.

鉴于上述规则，我们可以举例说明所涉及的语义：

Given the above rules, we can make an example of the semantics involved:

var o = {
  toString: function () { return "foo"; },
  valueOf:  function () { return "bar"; }
};

String(o); // "foo"

// Make the toString method unavailable:
o.toString = null;

String(o); // "bar"

// Also make the valueOf method unavailable:
o.valueOf = null;

try { 
  String(o); 
} catch (e) {
  alert(e); // TypeError
}

如果您想了解更多有关此机制的信息，我建议您查看在 ToPrimitive 和 ToString 内部操作。

If you want to know more about this mechanism I would recommend looking at the ToPrimitive and the ToString internal operations.

我还建议阅读这篇文章：

I also recommend reading this article:

• JavaScript中的对象到原始的转换

• Object-to-Primitive Conversions in JavaScript

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