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|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|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|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.

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|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|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|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>

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。

文章

问答

视频

学习中心

腾讯云实验室

直播

竞赛

腾讯云代码分析专区

腾讯iOA零信任安全管理系统专区

腾讯云架构师技术同盟交流圈

腾讯云数据库专区

腾讯云顾问专区

腾讯云原生专区

腾讯混元专区

腾讯云TCE专区

腾讯云Lighthouse专区

腾讯云HAI专区

腾讯云Edgeone专区

腾讯云存储专区

腾讯云智能专区

腾讯轻联专区 

腾讯云开发专区

TAPD专区

腾讯轻量云游戏服专区

腾讯云最具价值专家

腾讯云架构师技术同盟

腾讯云创作之星

腾讯云开发者先锋

腾讯云代码助手

云原生构建

TAPD 敏捷项目管理

Cloud Studio

SDK中心

API中心

命令行工具

涵盖代码开发、场景应用、自动测试全流程，助你从零构建专属AI助手

一站式MCP教程库，解锁AI应用新玩法

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

Stack Overflow用户

社区

活动

圈层

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问仅使用var在JavaScript中声明布尔值EN