blocks|key|1559547|text|在没有new的情况下调用全局函数Boolean()时，可以将其用于类型转换，例如|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1559548|var+foo+=+Boolean(bar);+//+equivalent+to+`var+foo+=+!!bar`|code-block|syntax|javascript|1559549|当使用new调用时，将额外创建一个包装器对象，这意味着您可以为该对象分配任意属性：|1559550|var+foo+=+new+Boolean(bar);+//+equivalent+to+`var+foo+=+Object(Boolean(bar));`
foo.baz+=+'quux';
alert(foo.baz);|1559551|对于原始值，这是不可能的，因为原语不能保存属性：|1559552|var+foo+=+true;
foo.baz+=+'quux';
alert(foo.baz);+//+`foo.baz`+is+`undefined`|1559553|将属性分配给原语不会因为自动装箱而产生错误，例如|1559554|foo.baz+=+'quux';|1559555|将被解释为|1559556|//+create+and+immediately+discard+a+wrapper+object:
(new+Boolean(foo)).baz+=+'quux';|1559557|要取回原始值，必须调用valueOf()方法。如果您想实际使用包装值，这是必需的，因为在布尔上下文中，对象的计算结果总是为true+-即使包装值为false。|1559558|我从来没有遇到过能够将属性分配给布尔值的有用应用程序，但是在需要引用原始值的情况下，装箱可能会很有用。|1559559|entityMap^0|3|3|G|9|0|0|3|3|0|0|0|0|0|0|0|0|B|9|1P|4|21|5|0|0^^$0|@$1|2|3|4|5|6|7|16|8|@$9|17|A|18|B|C]|$9|19|A|1A|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|1B|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|1C|8|@$9|1D|A|1E|B|C]]|D|@]|E|$]]|$1|M|3|N|5|H|7|1F|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|1G|8|@]|D|@]|E|$]]|$1|Q|3|R|5|H|7|1H|8|@]|D|@]|E|$I|J]]|$1|S|3|T|5|6|7|1I|8|@]|D|@]|E|$]]|$1|U|3|V|5|H|7|1J|8|@]|D|@]|E|$I|J]]|$1|W|3|X|5|6|7|1K|8|@]|D|@]|E|$]]|$1|Y|3|Z|5|H|7|1L|8|@]|D|@]|E|$I|J]]|$1|10|3|11|5|6|7|1M|8|@$9|1N|A|1O|B|C]|$9|1P|A|1Q|B|C]|$9|1R|A|1S|B|C]]|D|@]|E|$]]|$1|12|3|13|5|6|7|1T|8|@]|D|@]|E|$]]|$1|14|3|-4|5|6|7|1U|8|@]|D|@]|E|$]]]|15|$]]

The global function <code>Boolean()</code> can be used for type casting when called without <code>new</code>, eg

<pre><code>var foo = Boolean(bar); // equivalent to `var foo = !!bar`
</code></pre>

When called with <code>new</code>, a wrapper object will be created additionally, which means that you can assign arbitrary properties to the object:

<pre><code>var foo = new Boolean(bar); // equivalent to `var foo = Object(Boolean(bar));`
foo.baz = 'quux';
alert(foo.baz);
</code></pre>

This is not possible with primitive values as primitives can't hold properties:

<pre><code>var foo = true;
foo.baz = 'quux';
alert(foo.baz); // `foo.baz` is `undefined`
</code></pre>

Assigning a property to a primitive doesn't produce an error because of auto-boxing, ie

<pre><code>foo.baz = 'quux';
</code></pre>

will be interpreted as

<pre><code>// create and immediately discard a wrapper object:
(new Boolean(foo)).baz = 'quux';
</code></pre>

To get the primitive value back, you'll have to invoke the <code>valueOf()</code> method. This is needed if you want to actually use the wrapped value, because objects always evaluate to <code>true</code> in boolean contexts - even if the wrapped value is <code>false</code>.

I've never come across a useful application of being able to assign properties to booleans, but boxing might be useful in cases where a reference to a primitive value is needed.

blocks|key|1344061|text|虽然其他人提到了这个理论，但让我来谈谈实践部分：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1344062|因为Boolean对象(通常作为对象)总是真实的，所以使用它们被认为是不好的做法。在多年的JS编程中，我从未使用过它们，我也不记得在别人的代码中见过Boolean。一次也没有。|offset|length|style|CODE|1344063|使用原始值将避免混淆，并使您的代码更短。|1344064|如果你需要一个布尔包在一个对象中，你可以像这样使用一个Object对象：|1344065|foo+=+{+value:+false+};|code-block|syntax|javascript|1344066|此外，将Boolean()构造函数作为函数调用(就像在foo+=+Boolean(bar)中一样)与使用!!进行显式类型转换具有相同的效果，而且后者通常比前者更受欢迎。|1344067|entityMap^0|0|2|7|22|7|0|0|R|6|0|0|4|9|R|I|1G|2|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|V|8|@$D|W|E|X|F|G]|$D|Y|E|Z|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|10|8|@]|9|@]|A|$]]|$1|J|3|K|5|6|7|11|8|@$D|12|E|13|F|G]]|9|@]|A|$]]|$1|L|3|M|5|N|7|14|8|@]|9|@]|A|$O|P]]|$1|Q|3|R|5|6|7|15|8|@$D|16|E|17|F|G]|$D|18|E|19|F|G]|$D|1A|E|1B|F|G]]|9|@]|A|$]]|$1|S|3|-4|5|6|7|1C|8|@]|9|@]|A|$]]]|T|$]]

While others mentioned the theory, let me talk about the practical part:

Because <code>Boolean</code> objects (as objects in general) are always truthy, it is considered bad practice to use them. In many years of JS programming, I have never used them, and I can't remember seeing <code>Boolean</code>s in other peoples' code either. Not even once.

Using primitive values will avoid confusion and will make your code a little bit shorter.

If you ever need a bool wrapped in an object, you might as well use an <code>Object</code> object like so:

<pre><code>foo = { value: false };
</code></pre>

Also, calling the <code>Boolean()</code> constructor as a function (as in <code>foo = Boolean(bar)</code>) has the same effect as explicit typecasting using <code>!!</code>, and the latter is generally preferred over the former.

blocks|key|1559760|text|有趣的问题：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1559761|使用new+Boolean创建一个boolean对象。可能有很多种情况，但我已经讨论了下面的一种情况。|1559762|假设你想在你的代码中进行一个比较，你想要匹配字符串值和它的数据类型，并且它必须是bool+(true/+false+)，那么你将使用新的布尔值而不是分配简单的false值。|1559763|var+flag+=+false;++
var+flag2+=+new+Boolean+(false);
alert(typeof+flag);++//boolean+object
alert(typeof+flag2);+//simple+object

if+(flag+===+flag2){
++++alert("Value+and+datatype+match");
}
else{
++++alert("Value+and+datatype+do+not+match");
}|code-block|syntax|javascript|1559764|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|N|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|O|8|@]|9|@]|A|$]]|$1|F|3|G|5|H|7|P|8|@]|9|@]|A|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

Interesting question:

You use new Boolean to create a boolean object. There can be many scenarios but I have discussed below one scenario. 

Suppose you want a comparison in your code where you want to match string value and its datatype and it has to bool (true/false) then you will use new boolean instead of assigning simple false value.

<pre><code>var flag = false; 
var flag2 = new Boolean (false);
alert(typeof flag); //boolean object
alert(typeof flag2); //simple object

if (flag === flag2){
 alert("Value and datatype match");
}
else{
 alert("Value and datatype do not match");
}
</code></pre>

blocks|key|1344286|text|在上面的问题之前，首先是布尔函数Boolean+()|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1344287|Boolean(10+>+4)+//+return+true
Boolean(4+>+9)+//+return+false|code-block|syntax|javascript|1344288|下一步:所有有实值的东西都返回true。E.g|1344289|100
-4
4.4
"hello"
"false"+//+note+even+the+string+value+false+return+true.|1344290|所有没有实值的东西都返回false，例如|1344291|NaN+
var+x+=+10+/+"H";+//+Boolean(x);+return+false.
undefined
""+
0
-0
false+
null|1344292|现在，Boolean对象是布尔值的对象包装器。如果需要，作为第一个参数传递的值将转换为布尔值。如果省略value或为0,+-0,+null,+false,+NaN,+undefined，或者为空字符串("")，则对象的初始值为false。所有其他值，包括任何对象或字符串"false"，都会创建一个初始值为true的对象。|1344293|这允许使用非常强大的技巧。|1344294|entityMap^0|G|A|0|0|0|0|0|0|3|7|1M|Y|2T|2|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|11|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|12|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|13|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|14|8|@]|D|@]|E|$]]|$1|Q|3|R|5|H|7|15|8|@]|D|@]|E|$I|J]]|$1|S|3|T|5|6|7|16|8|@$9|17|A|18|B|C]|$9|19|A|1A|B|C]|$9|1B|A|1C|B|C]]|D|@]|E|$]]|$1|U|3|V|5|6|7|1D|8|@]|D|@]|E|$]]|$1|W|3|-4|5|6|7|1E|8|@]|D|@]|E|$]]]|X|$]]

Before the above question first the Boolean function, <code>Boolean ()</code>

<pre><code>Boolean(10 &gt; 4) // return true
Boolean(4 &gt; 9) // return false
</code></pre>

Next: everything with real value return true. E.g

<pre><code>100
-4
4.4
"hello"
"false" // note even the string value false return true.
</code></pre>

everthing without real value return false E.g 

<pre><code>NaN 
var x = 10 / "H"; // Boolean(x); return false.
undefined
"" 
0
-0
false 
null
</code></pre>

Now the <code>Boolean</code> object is an object wrapper for a boolean value. The value passed as the first parameter is converted to a boolean value, if necessary. If value is omitted or is <code>0, -0, null, false, NaN, undefined</code>, or the empty string (<code>""</code>), the object has an initial value of false. All other values, including any object or the string "false", create an object with an initial value of true.

This allows very powerful tricks.

What is the use of:

<pre><code>var flag = new Boolean(false); 
</code></pre>

compared to:

<pre><code>var flag = false;
</code></pre>

When would you actually use <code>new Boolean</code>?

What is the purpose of new Boolean() in Javascript?

以下命令有什么用途：var flag = new Boolean(false); 对比：var flag = false;什么时候会真正使用new Boolean？

问Javascript中new Boolean()的用途是什么？
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