Javascript new Number() 与 Number()

new Number() vs Number()

提问by Tom Tucker

What is the difference between new Number()and Number()? I get that new Number()creates a Numberobject and Number()is just a function, but when should I call which, and why?

new Number()和 和有Number()什么区别？我知道它new Number()创建了一个Number对象并且Number()只是一个函数，但是我什么时候应该调用 which，为什么？

On a related note, Mozilla says:

在相关说明中，Mozilla 说：

Do not use a Boolean object to convert a non-boolean value to a boolean value. Instead, use Boolean as a function to perform this task.

x = Boolean(expression);     // preferred
x = new Boolean(expression); // don't use

不要使用布尔对象将非布尔值转换为布尔值。相反，使用 Boolean 作为函数来执行此任务。

x = Boolean(expression);     // preferred
x = new Boolean(expression); // don't use

Why is that? I thought the results were the same?

这是为什么？我以为结果是一样的？

采纳答案by David Tang

Boolean(expression)will simply convert the expression into a boolean primitive value, while new Boolean(expression)will create a wrapper objectaround the converted boolean value.

Boolean(expression)将简单地将表达式转换为布尔原始值，同时new Boolean(expression)将围绕转换后的布尔值创建一个包装对象。

The difference can be seen with this:

可以看出不同之处：

// Note I'm using strict-equals
new Boolean("true") === true; // false
Boolean("true") === true; // true

And also with this (thanks @hobbs):

还有这个（感谢@hobbs）：

typeof new Boolean("true"); // "object"
typeof Boolean("true"); // "boolean"

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Note:While the wrapper object will get converted to the primitive automatically when necessary (and vice versa), there is only one case I can think of where you would want to use new Boolean, or any of the other wrappers for primitives - if you want to attach properties to a single value. E.g:

注意：虽然包装器对象会在必要时自动转换为原语（反之亦然），但我只能想到一种情况，你想在哪里使用new Boolean，或任何其他用于原语的包装器 - 如果你想将属性附加到单个值。例如：

var b = new Boolean(true);
b.relatedMessage = "this should be true initially";
alert(b.relatedMessage); // will work

var b = true;
b.relatedMessage = "this should be true initially";
alert(b.relatedMessage); // undefined

回答by ?ime Vidas

new Number( x )

creates a new wrapper object. I don't think that there is a valid reason to ever use this.

创建一个新的包装对象。我不认为有充分的理由使用它。

Number( x )

converts the passed argument into a Number value. You can use this to cast some variable to the Number type. However this gets the same job done:

将传递的参数转换为 Number 值。您可以使用它来将一些变量转换为 Number 类型。但是，这可以完成相同的工作：

+x

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Generally:

一般来说：

You don't need those:

你不需要那些：

new Number()
new String()
new Boolean()

You can use those for casting:

您可以将它们用于铸造：

Number( value )
String( value )
Boolean( value )

However, there are simpler solutions for casting:

但是，有更简单的铸造解决方案：

+x // cast to Number
'' + x // cast to String
!!x // cast to Boolean

回答by T.J. Crowder

Always worth consulting the spec; from Section 15.7.1:

始终值得咨询规范；来自第 15.7.1 节：

When Numberis called as a function rather than as a constructor, it performs a type conversion.

当Number作为函数而不是构造函数调用时，它执行类型转换。

Similarly, using Booleanas a function (15.6.1):

同样，Boolean用作函数（15.6.1）：

When Boolean is called as a function rather than as a constructor, it performs a type conversion.

当 Boolean 作为函数而不是构造函数调用时，它执行类型转换。

...which means that you consult Section 9.2 ("ToBoolean"):

...这意味着您查阅第 9.2 节（“ToBoolean”）：

The abstract operation ToBoolean converts its argument to a value of type Boolean according to Table 11:
Undefined= false
Null= false
Boolean= The result equals the input argument (no conversion).
Number= The result is false if the argument is +0, ?0, or NaN; otherwise the result is true.
String= The result is false if the argument is the empty String (its length is zero); otherwise the result is true.
Object= true

抽象操作 ToBoolean 根据表 11 将其参数转换为 Boolean 类型的值：
Undefined== false
Null= false
Boolean结果等于输入参数（无转换）。
Number= 如果参数为 +0、?0 或 NaN，则结果为 false；否则结果为真。
String= 如果参数为空字符串（其长度为零），则结果为假；否则结果为真。
Object=true

The difference between new Boolean(value)and Boolean(value)is basically that the former returns an object, but the latter returns a primitive per the above. This matters, because objects are truthy:

new Boolean(value)和之间的区别Boolean(value)基本上是前者返回一个对象，而后者按照上述返回一个原语。这很重要，因为对象是真实的：

var b = new Boolean(false);

display(b);            // Displays "false"
if (b) {
  display("true");     // This is the path that gets taken, displaying "true"
}
else {
  display("false");    // This path does NOT get taken
}

Live example...whereas you almost always want booleans for the purpose of testing them.

现场示例...而您几乎总是想要布尔值来测试它们。