blocks|key|4544334|text|类型取决于您的初始化：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4544335|var+IsLoggedIn1+=+"true";+//string
var+IsLoggedIn2+=+1;+//integer
var+IsLoggedIn3+=+true;+//bool|code-block|syntax|javascript|4544336|但是看看这个例子：|4544337|var+IsLoggedIn1+=+"true";+//string
IsLoggedIn1+=+true;+//now+your+variable+is+a+boolean|4544338|变量的类型取决于JavaScript中的赋值。|4544339|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

Types are dependent to your initialization:

<pre><code>var IsLoggedIn1 = "true"; //string
var IsLoggedIn2 = 1; //integer
var IsLoggedIn3 = true; //bool
</code></pre>

But take a look at this example: 

<pre><code>var IsLoggedIn1 = "true"; //string
IsLoggedIn1 = true; //now your variable is a boolean
</code></pre>

Your variables' type depends on the assigned value in JavaScript.

blocks|key|1100458|text|不不安全。您可以稍后执行var+IsLoggedIn+=+"Foo";，JavaScript不会抛出错误。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1100459|这样做是可能的|1100460|var+IsLoggedIn+=+new+Boolean(false);
var+IsLoggedIn+=+new+Boolean(true);|code-block|syntax|javascript|1100461|您还可以将非布尔变量传递给new+Boolean()，它将使IsLoggedIn成为布尔值。|1100462|var+IsLoggedIn+=+new+Boolean(0);+//+false
var+IsLoggedIn+=+new+Boolean(NaN);+//+false
var+IsLoggedIn+=+new+Boolean("Foo");+//+true
var+IsLoggedIn+=+new+Boolean(1);+//+true|1100463|entityMap^0|G|J|0|0|0|D|D|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|V|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|W|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|X|8|@$9|Y|A|Z|B|C]]|D|@]|E|$]]|$1|O|3|P|5|J|7|10|8|@]|D|@]|E|$K|L]]|$1|Q|3|-4|5|6|7|11|8|@]|D|@]|E|$]]]|R|$]]

No it is not safe. You could later do var <code>IsLoggedIn = "Foo";</code> and JavaScript will not throw an error.

It is possible to do

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

You can also pass the non boolean variable into the <code>new Boolean()</code> and it will make IsLoggedIn boolean. 

<pre><code>var IsLoggedIn = new Boolean(0); // false
var IsLoggedIn = new Boolean(NaN); // false
var IsLoggedIn = new Boolean("Foo"); // true
var IsLoggedIn = new Boolean(1); // true
</code></pre>

blocks|key|4544479|text|正如这个very+useful+tutorial所说的：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|4544480|var+age+=+0;

//+bad
var+hasAge+=+new+Boolean(age);

//+good
var+hasAge+=+Boolean(age);

//+good
var+hasAge+=+!!age;|code-block|syntax|javascript|4544481|entityMap|0|LINK|mutability|MUTABLE|url|https://github.com/airbnb/javascript^0|4|K|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@$A|R|B|S|1|T]]|C|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|C|$G|H]]|$1|I|3|-4|5|6|7|V|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]]]

As this <a href="https://github.com/airbnb/javascript" rel="noreferrer">very useful tutorial</a> says:

<pre><code>var age = 0;

// bad
var hasAge = new Boolean(age);

// good
var hasAge = Boolean(age);

// good
var hasAge = !!age;
</code></pre>

blocks|key|478816|text|如果您希望将IsLoggedIn视为布尔值，则应按如下方式进行初始化：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|478817|var+IsLoggedIn=true;|code-block|syntax|javascript|478818|如果你用var+IsLoggedIn=1;初始化它，那么它将被视为一个整数。|478819|但是，在任何时候，变量IsLoggedIn都可以引用不同的数据类型：|478820|+IsLoggedIn="Hello+World";|478821|这不会导致错误。|478822|entityMap^0|6|A|0|0|4|H|0|B|A|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|X|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|Y|8|@$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|M|3|N|5|6|7|11|8|@$9|12|A|13|B|C]]|D|@]|E|$]]|$1|O|3|P|5|H|7|14|8|@]|D|@]|E|$I|J]]|$1|Q|3|R|5|6|7|15|8|@]|D|@]|E|$]]|$1|S|3|-4|5|6|7|16|8|@]|D|@]|E|$]]]|T|$]]

If you want <code>IsLoggedIn</code> to be treated as a boolean you should initialize as follows:

<pre><code>var IsLoggedIn=true;
</code></pre>

If you initialize it with <code>var IsLoggedIn=1;</code> then it will be treated as an integer.

However at any time the variable <code>IsLoggedIn</code> could refer to a different data type:

<pre><code> IsLoggedIn="Hello World";
</code></pre>

This will not cause an error.

blocks|key|1100530|text|你可以使用和测试未初始化的变量，至少测试它们的“定义”。如下所示：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1100531|var+iAmNotDefined;
alert(!iAmNotDefined);+//true
//or
alert(!!iAmNotDefined);+//false|code-block|syntax|javascript|1100532|此外，还有许多可能性:如果您对精确类型不感兴趣，请使用'==‘运算符(或/+!)作为比较(我认为这就是道格拉斯·克罗克福德所说的“真实”或“虚假”)。在这种情况下，如果将true、1或'1‘赋值给实例化的变量，则在被要求时总是返回true。否则，如果您需要类型安全比较，请使用'===‘进行比较。|1100533|var+thisMayBeTrue;

thisMayBeTrue+=+1;
alert(thisMayBeTrue+==+true);+//=>+true
alert(!!thisMayBeTrue);+//=>+true
alert(thisMayBeTrue+===+true);+//=>+false

thisMayBeTrue+=+'1';
alert(thisMayBeTrue+==+true);+//=>+true+
alert(!!thisMayBeTrue);+//=>+true
alert(thisMayBeTrue+===+true);+//=>+false
//+so,+in+this+case,+using+==+or+!!+'1'+is+implicitly+
//+converted+to+1+and+1+is+implicitly+converted+to+true)

thisMayBeTrue+=+true;
alert(thisMayBeTrue+==+true);+//=>+true
alert(!!thisMayBeTrue);+//=>+true
alert(thisMayBeTrue+===+true);+//=>+true

thisMayBeTrue+=+'true';
alert(thisMayBeTrue+==+true);+//=>+false
alert(!!thisMayBeTrue);+//=>+true
alert(thisMayBeTrue+===+true);+//=>+false
//+so,+here's+no+implicit+conversion+of+the+string+'true'
//+it's+also+a+demonstration+of+the+fact+that+the+
//+!+or+!!+operator+tests+the+'definedness'+of+a+variable.|1100534|PS:你不能对不存在的变量进行“定义”测试。所以：|1100535|alert(!!HelloWorld);|1100536|给出一个引用错误('HelloWorld+is+not+defined')|1100537|(有没有更好的词来形容“定义”？无论如何请原谅我的荷兰语；~)|1100538|entityMap^0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|W|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|X|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|Y|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|Z|8|@]|9|@]|A|$E|F]]|$1|O|3|P|5|6|7|10|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|11|8|@]|9|@]|A|$]]|$1|S|3|-4|5|6|7|12|8|@]|9|@]|A|$]]]|T|$]]

You can use and test uninitialized variables at least for their 'definedness'. 
Like this:

<pre><code>var iAmNotDefined;
alert(!iAmNotDefined); //true
//or
alert(!!iAmNotDefined); //false
</code></pre>

Furthermore, there are many possibilites: if you're not interested in exact types use the '==' operator (or ![variable] / !![variable]) for comparison (that is what Douglas Crockford calls 'truthy' or 'falsy' I think). In that case assigning true or 1 or '1' to the unitialized variable always returns true when asked. Otherwise [if you need type safe comparison] use '===' for comparison.

<pre><code>var thisMayBeTrue;

thisMayBeTrue = 1;
alert(thisMayBeTrue == true); //=&gt; true
alert(!!thisMayBeTrue); //=&gt; true
alert(thisMayBeTrue === true); //=&gt; false

thisMayBeTrue = '1';
alert(thisMayBeTrue == true); //=&gt; true 
alert(!!thisMayBeTrue); //=&gt; true
alert(thisMayBeTrue === true); //=&gt; false
// so, in this case, using == or !! '1' is implicitly 
// converted to 1 and 1 is implicitly converted to true)

thisMayBeTrue = true;
alert(thisMayBeTrue == true); //=&gt; true
alert(!!thisMayBeTrue); //=&gt; true
alert(thisMayBeTrue === true); //=&gt; true

thisMayBeTrue = 'true';
alert(thisMayBeTrue == true); //=&gt; false
alert(!!thisMayBeTrue); //=&gt; true
alert(thisMayBeTrue === true); //=&gt; false
// so, here's no implicit conversion of the string 'true'
// it's also a demonstration of the fact that the 
// ! or !! operator tests the 'definedness' of a variable.
</code></pre>

PS: you can't test 'definedness' for nonexisting variables though. So:

<pre><code>alert(!!HelloWorld);
</code></pre>

gives a reference Error ('HelloWorld is not defined')

(is there a better word for 'definedness'? Pardon my dutch anyway;~)

blocks|key|3698344|text|Javascript中的变量没有类型。非零、非空、非空和true为“真”。0、null、undefined、空字符串和false为false。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3698345|这里有一个布尔类型，就像文字true和false一样。|3698346|entityMap^0|S|4|1N|5|0|E|4|J|5|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]|$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|O|8|@$9|P|A|Q|B|C]|$9|R|A|S|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|T|8|@]|D|@]|E|$]]]|I|$]]

Variables in Javascript don't have a type. Non-zero, non-null, non-empty and <code>true</code> are "true". Zero, null, undefined, empty string and <code>false</code> are "false".

There's a Boolean type though, as are literals <code>true</code> and <code>false</code>.

blocks|key|4544449|text|这样如何：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4544450|var+MyNamespace+=+{
++++convertToBoolean:+function+(value)+{
++++++++//VALIDATE+INPUT
++++++++if+(typeof+value+===+'undefined'+%7C%7C+value+===+null)+return+false;

++++++++//DETERMINE+BOOLEAN+VALUE+FROM+STRING
++++++++if+(typeof+value+===+'string')+{
++++++++++++switch+(value.toLowerCase())+{
++++++++++++++++case+'true':
++++++++++++++++case+'yes':
++++++++++++++++case+'1':
++++++++++++++++++++return+true;
++++++++++++++++case+'false':
++++++++++++++++case+'no':
++++++++++++++++case+'0':
++++++++++++++++++++return+false;
++++++++++++}
++++++++}

++++++++//RETURN+DEFAULT+HANDLER
++++++++return+Boolean(value);
++++}
};|code-block|syntax|javascript|4544451|然后你可以像这样使用它：|4544452|MyNamespace.convertToBoolean('true')+//true
MyNamespace.convertToBoolean('no')+//false
MyNamespace.convertToBoolean('1')+//true
MyNamespace.convertToBoolean(0)+//false|4544453|我还没有测试它的性能，但是从类型到类型的转换不应该发生得太频繁，否则你打开你的应用程序的时候就会很不稳定！|4544454|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

How about something like this:

<pre><code>var MyNamespace = {
 convertToBoolean: function (value) {
 //VALIDATE INPUT
 if (typeof value === 'undefined' || value === null) return false;

 //DETERMINE BOOLEAN VALUE FROM STRING
 if (typeof value === 'string') {
 switch (value.toLowerCase()) {
 case 'true':
 case 'yes':
 case '1':
 return true;
 case 'false':
 case 'no':
 case '0':
 return false;
 }
 }

 //RETURN DEFAULT HANDLER
 return Boolean(value);
 }
};
</code></pre>

Then you can use it like this:

<pre><code>MyNamespace.convertToBoolean('true') //true
MyNamespace.convertToBoolean('no') //false
MyNamespace.convertToBoolean('1') //true
MyNamespace.convertToBoolean(0) //false
</code></pre>

I have not tested it for performance, but converting from type to type should not happen too often otherwise you open your app up to instability big time!

blocks|key|3698331|text|该变量将成为您为其指定的任何类型。最初它是undefined。如果你给它赋值'true'，它会变成一个字符串；如果你给它赋值true，它会变成一个布尔值；如果你给它赋值1，它会变成一个数字。随后的赋值可能会在以后更改变量的类型。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3698332|entityMap^0|L|9|12|6|1Q|4|2C|1|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]|$9|M|A|N|B|C]|$9|O|A|P|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|G|$]]

The variable will become what ever type you assign it. Initially it is <code>undefined</code>. If you assign it <code>'true'</code> it will become a string, if you assign it <code>true</code> it will become a boolean, if you assign it <code>1</code> it will become a number. Subsequent assignments may change the type of the variable later.

If I declare a JavaScript boolean variable like this:

<pre><code>var IsLoggedIn;
</code></pre>

And then initialize it with either <code>true</code> or <code>1</code>, is that safe? Or will initializing it with <code>1</code> make the variable a number?

Declaring a boolean in JavaScript using just var

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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如果我像这样声明一个JavaScript布尔变量：var IsLoggedIn;然后用true或1初始化它，这样安全吗？或者用1初始化它会使变量变成一个数字？

问仅使用var在JavaScript中声明布尔值
EN

回答 8

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问仅使用var在JavaScript中声明布尔值
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