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JS中typeof与instanceof的区别

2017-02-16 16:16 507 查看
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JavaScript 中 typeof 和 instanceof 常用来判断一个变量是否为空，或者是什么类型的。但它们之间还是有区别的：<br>
typeof<br>
<span style="color:#000099">typeof 是一个一元运算，放在一个运算数之前，运算数可以是任意类型。</span><br>
它返回值是一个字符串，该字符串说明运算数的类型。typeof 一般只能返回如下几个结果：<br>
<span style="background-color:rgb(0,0,153)">number,boolean,string,function,object,undefined。</span>我们可以使用 typeof 来获取一个变量是否存在，如 if(typeof a!="undefined"){alert("ok")}，而不要去使用 if(a) 因为如果 a 不存在（未声明）则会出错，对于 Array,Null 等特殊对象使用 typeof 一律返回 object，这正是 typeof 的局限性。<br>
<p>网上的一个小例子：</p>
<p></p><div class="dp-highlighter bg_html"><div class="bar"><div class="tools"><b>[html]</b> <a href="#" class="ViewSource" title="view plain" onclick="dp.sh.Toolbar.Command('ViewSource',this);return false;">view plain</a><span data-mod="popu_168"> <a href="#" class="CopyToClipboard" title="copy" onclick="dp.sh.Toolbar.Command('CopyToClipboard',this);return false;">copy</a><div style="position: absolute; left: 500px; top: 677px; width: 24px; height: 13px; z-index: 99;"><embed id="ZeroClipboardMovie_1" src="http://static.blog.csdn.net/scripts/ZeroClipboard/ZeroClipboard.swf" loop="false" menu="false" quality="best" bgcolor="#ffffff" name="ZeroClipboardMovie_1" allowscriptaccess="always" allowfullscreen="false" type="application/x-shockwave-flash" pluginspage="http://www.macromedia.com/go/getflashplayer" flashvars="id=1&width=24&height=13" wmode="transparent" align="middle" height="13" width="24"></div></span><span data-mod="popu_169"> <a href="#" class="PrintSource" title="print" onclick="dp.sh.Toolbar.Command('PrintSource',this);return false;">print</a></span><a href="#" class="About" title="?" onclick="dp.sh.Toolbar.Command('About',this);return false;">?</a><span class="tracking-ad" data-mod="popu_167"><a href="https://code.csdn.net/snippets/2157718" target="_blank" title="在CODE上查看代码片" style="text-indent:0;"><img src="https://code.csdn.net/assets/CODE_ico.png" alt="在CODE上查看代码片" style="position:relative;top:1px;left:2px;" height="12" width="12"></a></span><span class="tracking-ad" data-mod="popu_170"><a href="https://code.csdn.net/snippets/2157718/fork" target="_blank" title="派生到我的代码片" style="text-indent:0;"><img src="https://code.csdn.net/assets/ico_fork.svg" alt="派生到我的代码片" style="position:relative;top:2px;left:2px;" height="12" width="12"></a></span></div></div><ol start="1" class="dp-xml"><li class="alt"><span><span><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"</span><span class="tag">></span><span>  </span></span></li><li class=""><span><span class="tag"><</span><span class="tag-name">html</span><span> </span><span class="attribute">xmlns</span><span>=</span><span class="attribute-value">"http://www.w3.org/1999/xhtml"</span><span class="tag">></span><span>  </span></span></li><li class="alt"><span><span class="tag"><</span><span class="tag-name">head</span><span class="tag">></span><span>  </span></span></li><li class=""><span><span class="tag"><</span><span class="tag-name">meta</span><span> </span><span class="attribute">http-equiv</span><span>=</span><span class="attribute-value">"Content-Type"</span><span> </span><span class="attribute">content</span><span>=</span><span class="attribute-value">"text/html; charset=utf-8"</span><span> </span><span class="tag">/></span><span>  </span></span></li><li class="alt"><span><span class="tag"><</span><span class="tag-name">script</span><span> </span><span class="attribute">language</span><span>=</span><span class="attribute-value">"javascript"</span><span> </span><span class="attribute">type</span><span>=</span><span class="attribute-value">"text/javascript"</span><span class="tag">></span><span>  </span></span></li><li class=""><span>document.write ("typeof(1): "+typeof(1)+"<span class="tag"><</span><span class="tag-name">br</span><span class="tag">></span><span>");  </span></span></li><li class="alt"><span>document.write ("typeof(NaN): "+typeof(NaN)+"<span class="tag"><</span><span class="tag-name">br</span><span class="tag">></span><span>");  </span></span></li><li class=""><span>document.write ("typeof(Number.MIN_VALUE): "+typeof(Number.MIN_VALUE)+"<span class="tag"><</span><span class="tag-name">br</span><span class="tag">></span><span>");  </span></span></li><li class="alt"><span>document.write ("typeof(Infinity): "+typeof(Infinity)+"<span class="tag"><</span><span class="tag-name">br</span><span class="tag">></span><span>");  </span></span></li><li class=""><span>document.write ("typeof(\"123\"): "+typeof("123")+"<span class="tag"><</span><span class="tag-name">br</span><span class="tag">></span><span>");  </span></span></li><li class="alt"><span>document.write ("typeof(true): "+typeof(true)+"<span class="tag"><</span><span class="tag-name">br</span><span class="tag">></span><span>");  </span></span></li><li class=""><span>document.write ("typeof(window): "+typeof(window)+"<span class="tag"><</span><span class="tag-name">br</span><span class="tag">></span><span>");  </span></span></li><li class="alt"><span>document.write ("typeof(Array()): "+typeof(new Array())+"<span class="tag"><</span><span class="tag-name">br</span><span class="tag">></span><span>");  </span></span></li><li class=""><span>document.write ("typeof(function(){}): "+typeof(function(){})+"<span class="tag"><</span><span class="tag-name">br</span><span class="tag">></span><span>");  </span></span></li><li class="alt"><span>document.write ("typeof(document): "+typeof(document)+"<span class="tag"><</span><span class="tag-name">br</span><span class="tag">></span><span>");  </span></span></li><li class=""><span>document.write ("typeof(null): "+typeof(null)+"<span class="tag"><</span><span class="tag-name">br</span>&
4000
lt;span class="tag">></span><span>");  </span></span></li><li class="alt"><span>document.write ("typeof(eval): "+typeof(eval)+"<span class="tag"><</span><span class="tag-name">br</span><span class="tag">></span><span>");  </span></span></li><li class=""><span>document.write ("typeof(Date): "+typeof(Date)+"<span class="tag"><</span><span class="tag-name">br</span><span class="tag">></span><span>");  </span></span></li><li class="alt"><span>document.write ("typeof(sss): "+typeof(sss)+"<span class="tag"><</span><span class="tag-name">br</span><span class="tag">></span><span>");  </span></span></li><li class=""><span>document.write ("typeof(undefined): "+typeof(undefined)+"<span class="tag"><</span><span class="tag-name">br</span><span class="tag">></span><span>")  </span></span></li><li class="alt"><span><span class="tag"></</span><span class="tag-name">script</span><span class="tag">></span><span>  </span></span></li><li class=""><span><span class="tag"><</span><span class="tag-name">title</span><span class="tag">></span><span>javascript类型测试</span><span class="tag"></</span><span class="tag-name">title</span><span class="tag">></span><span>  </span></span></li><li class="alt"><span><span class="tag"></</span><span class="tag-name">head</span><span class="tag">></span><span>  </span></span></li><li class=""><span>  </span></li><li class="alt"><span><span class="tag"><</span><span class="tag-name">body</span><span class="tag">></span><span>  </span></span></li><li class=""><span><span class="tag"></</span><span class="tag-name">body</span><span class="tag">></span><span>  </span></span></li><li class="alt"><span><span class="tag"></</span><span class="tag-name">html</span><span class="tag">></span><span>  </span></span></li></ol><div class="save_code tracking-ad" data-mod="popu_249" style="display: none;"><a href="javascript:;" target="_blank"><img src="http://static.blog.csdn.net/images/save_snippets.png"></a></div></div><pre code_snippet_id="2157718" snippet_file_name="blog_20170204_1_7658360" name="code" class="html" style="display: none;"><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<script language="javascript" type="text/javascript">
document.write ("typeof(1): "+typeof(1)+"<br>");
document.write ("typeof(NaN): "+typeof(NaN)+"<br>");
document.write ("typeof(Number.MIN_VALUE): "+typeof(Number.MIN_VALUE)+"<br>");
document.write ("typeof(Infinity): "+typeof(Infinity)+"<br>");
document.write ("typeof(\"123\"): "+typeof("123")+"<br>");
document.write ("typeof(true): "+typeof(true)+"<br>");
document.write ("typeof(window): "+typeof(window)+"<br>");
document.write ("typeof(Array()): "+typeof(new Array())+"<br>");
document.write ("typeof(function(){}): "+typeof(function(){})+"<br>");
document.write ("typeof(document): "+typeof(document)+"<br>");
document.write ("typeof(null): "+typeof(null)+"<br>");
document.write ("typeof(eval): "+typeof(eval)+"<br>");
document.write ("typeof(Date): "+typeof(Date)+"<br>");
document.write ("typeof(sss): "+typeof(sss)+"<br>");
document.write ("typeof(undefined): "+typeof(undefined)+"<br>")
</script>
<title>javascript类型测试</title>
</head>

<body>
</body>
</html></pre>instanceof<br>
instance：实例,例子<br>
a instanceof b?alert("true"):alert("false"); //a是b的实例？真:假<br>
instanceof 用于判断一个变量是否某个对象的实例，如 var a=new Array();alert(a instanceof Array); 会返回 true，同时 alert(a instanceof Object) 也会返回 true;这是因为 Array 是 object 的子类。再如：function test(){};var a=new test();alert(a instanceof test) 会返回<br>
谈到 instanceof 我们要多插入一个问题，就是 function 的 arguments，我们大家也许都认为 arguments 是一个 Array，但如果使用 instaceof 去测试会发现 arguments 不是一个 Array 对象，尽管看起来很像。<br>
另外：<br>
测试 var a=new Array();if (a instanceof Object) alert('Y');else alert('N');<br>
得'Y’<br>
但 if (window instanceof Object) alert('Y');else alert('N');<br>
得'N'。<span style="color:#000099">（PS：经证实，目前主流浏览器都返回Y。）</span><br>
所以，这里的 instanceof 测试的 object 是指 js 语法中的 object，不是指 dom 模型对象。<br>
使用 typeof 会有些区别<br>
alert(typeof(window)) 会得 object<br>
<br>
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