the diff typeof and instanceof
2013-10-15 10:54
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instanceof和typeof都能用来判断一个变量是否为空或是什么类型的变量。 typeof用以获取一个变量的类型,typeof一般只能返回如下几个结果:number,boolean,string,function,object,undefined。我们可以使用typeof来获取一个变量是否存在,如if(typeof a!="undefined"){},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。 如果我们希望获取一个对象是否是数组,或判断某个变量是否是某个对象的实例则要选择使用instanceof。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true。 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。 另外: 测试 var a=new Array();if (a instanceof Object) alert('Y');else alert('N'); 得'Y’ 但 if (window instanceof Object) alert('Y');else alert('N'); 得'N' 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。 使用typeof会有些区别 alert(typeof(window) 会得 object
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