typeof 2 输出 number 
typeof null 输出 object 
typeof {} 输出 object 
typeof [] 输出 object 
typeof (function(){}) 输出 function 
typeof undefined 输出 undefined 
typeof '222' 输出 string 
typeof true 输出 boolean

  var c= [1,2,3]; 
  var d = new Date(); 
  var e = function(){alert(111);}; 
  var f = function(){this.name="22";}; 
  console.log(c instanceof Array) //true
  console.log(d instanceof Date) //true
  console.log(e instanceof Function) //true
  // console.log(f instanceof function ) //false

var c= [1,2,3]; 
var d = new Date(); 
var e = function(){alert(111);}; 
alert(c.constructor === Array) ----------> true 
alert(d.constructor === Date) -----------> true 
alert(e.constructor === Function) -------> true 
//注意： constructor 在类继承时会出错

var gettype=Object.prototype.toString
gettype.call('aaaa') 输出 [object String] 
gettype.call(2222) 输出 [object Number] 
gettype.call(true) 输出 [object Boolean] 
gettype.call(undefined) 输出 [object Undefined] 
gettype.call(null) 输出 [object Null] 
gettype.call({}) 输出 [object Object] 
gettype.call([]) 输出 [object Array] 
gettype.call(function(){}) 输出 [object Function]

var gettype=Object.prototype.toString 
var utility={ 
  isObj:function(o){ 
    return gettype.call(o)=="[object Object]"; 
  }, 
  isArray:function(o){ 
    return gettype.call(o)=="[object Array]"; 
  }, 
  isNULL:function(o){ 
    return gettype.call(o)=="[object Null]"; 
  }, 
  isDocument:function(){ 
    return gettype.call(o)=="[object Document]"|| [object HTMLDocument]; 
  } 
  ........ 
}