instanceof, typeof

instanceof, typeof

typeof 會返回的結果只有 number,boolean,string,function,object,undefined,
對於Array,Null等特殊對象使用typeof一律返回object，這正是typeof的侷限性。, 注意：undefined 是 undefined, null 是 object

而如果是Object的話, 可以用instanceof 進一步確認是什麼object
<br />>>> var f = function(){}<br />>>> typeof f == "function"<br />true<br />>>> f instanceof Function<br />true<br />>>> f instanceof Object<br />true<br />>>> f instanceof Array<br />false<br /><br />//---------------------<br />>>> var d = new Date();<br />>>> d instanceof Object;<br />true<br />>>> d.constructor instanceof Object;<br />true<br />>>> d.constructor == Object;<br />false<br />>>> d.constructor == Date;<br />true<br />

至於如何辨識他是什麼Object, 可以利用toString, 但是因為常常被overwrite 掉, 所以就必須依賴prototype, 不過這招也只有內建的型態才有用
<br />>>> d.toString()<br />"Wed Feb 18 2009 07:56:09 GMT+0800 (CST)"<br />>>> Object.prototype.toString.apply(d)<br />"[object Date]"<br /><br />//使用者自行定義的可以利用classname property<br /> // For user-defined classes, look for a string-valued property named<br /> // classname, that is inherited from the object's prototype <br /> if ("classname" in x.constructor.prototype && // inherits classname <br /> typeof x.constructor.prototype.classname == "string") <blockquote></blockquote>// its a string <br /> return x.constructor.prototype.classname; <br />