类型判断
返回类型：typeof
var o;
document.write( typeof o );     //结果：undefined

var s = "字符串";
document.write( typeof s );      //结果：string

var i = 100;
document.write( typeof i );      //结果：number

var bol = true;
document.write( typeof bol );    //结果：boolean

function fun(){}
document.write( typeof fun );    //结果：function
    
var arr = [1,2];
document.write( typeof arr );    //结果：object

var obj = {a:"a1", b:"b1"}
document.write( typeof obj );    //结果：object

var dt = new Date();
document.write( typeof dt );     //结果：object



注：typeof 对数组、时间类型，判断都是对象，最好用constructor返回构造函数

    
返回构造：constructor
    
var o;     // 注意：不能测试未赋值的变量
      
var s = "字符串";
document.write( s.constructor );  //结果：function String() { [native code] }
    
var i = 100;
document.write( i.constructor );  //结果：function Number() { [native code] }
     
var bol = true;
document.write( bol.constructor ); //结果：function Boolean() { [native code] }

function fun(){}
document.write( fun.constructor ); //结果：function Function() { [native code] }
  
var arr = [1,2];
document.write( arr.constructor ); //结果：function Array() { [native code] }

var obj = {a:"a1", b:"b1"}
document.write( obj.constructor ); //结果：function Object() { [native code] }
     
var dt = new Date();
document.write( dt.constructor );  //结果：function Date() { [native code] }
    

constructor判断方法:
if(s.constructor === String){
    // 是字符串
}