var num = 123;
var str = 'abcdef';
var bool = true;
var arr = [1, 2, 3, 4];
var json = {name:'wenzi', age:25};
var func = function(){ console.log('this is function'); }
var und = undefined;
var nul = null;
var date = new Date();
var reg = /^[a-zA-Z]{5,20}$/;
var error= new Error();

console.log(
 typeof num, 
 typeof str, 
 typeof bool, 
 typeof arr, 
 typeof json, 
 typeof func, 
 typeof und, 
 typeof nul, 
 typeof date, 
 typeof reg, 
 typeof error
);
// number string boolean object object function undefined object object object object

function Person(){

}
var Tom = new Person();
console.log(Tom instanceof Person); // true

function Person(){

}
function Student(){

}
Student.prototype = new Person();
var John = new Student();
console.log(John instanceof Student); // true
console.log(John instancdof Person); // true

console.log(
 num instanceof Number,
 str instanceof String,
 bool instanceof Boolean,
 arr instanceof Array,
 json instanceof Object,
 func instanceof Function,
 und instanceof Object,
 nul instanceof Object,
 date instanceof Date,
 reg instanceof RegExp,
 error instanceof Error
)
// num : false 
// str : false 
// bool : false 
// arr : true 
// json : true 
// func : true 
// und : false 
// nul : false 
// date : true 
// reg : true 
// error : true

var num = new Number(123);
var str = new String('abcdef');
var boolean = new Boolean(true);

function Person(){

}
var Tom = new Person();

// undefined和null没有constructor属性
console.log(
 Tom.constructor==Person,
 num.constructor==Number,
 str.constructor==String,
 bool.constructor==Boolean,
 arr.constructor==Array,
 json.constructor==Object,
 func.constructor==Function,
 date.constructor==Date,
 reg.constructor==RegExp,
 error.constructor==Error
);
// 所有结果均为true

function Person(){

}
function Student(){

}
Student.prototype = new Person();
var John = new Student();
console.log(John.constructor==Student); // false
console.log(John.constructor==Person); // true

console.log(
 Object.prototype.toString.call(num),
 Object.prototype.toString.call(str),
 Object.prototype.toString.call(bool),
 Object.prototype.toString.call(arr),
 Object.prototype.toString.call(json),
 Object.prototype.toString.call(func),
 Object.prototype.toString.call(und),
 Object.prototype.toString.call(nul),
 Object.prototype.toString.call(date),
 Object.prototype.toString.call(reg),
 Object.prototype.toString.call(error)
);
// '[object Number]' '[object String]' '[object Boolean]' '[object Array]' '[object Object]'
// '[object Function]' '[object Undefined]' '[object Null]' '[object Date]' '[object RegExp]' '[object Error]'

console.log(
 $.type(num),
 $.type(str),
 $.type(bool),
 $.type(arr),
 $.type(json),
 $.type(func),
 $.type(und),
 $.type(nul),
 $.type(date),
 $.type(reg),
 $.type(error)
);
// number string boolean array object function undefined null date regexp error

// 实例对象是能直接使用原型链上的方法的
var class2type = {};
var toString = class2type.toString;

// 省略部分代码...

type: function( obj ) {
 if ( obj == null ) {
  return obj + "";
 }
 // Support: Android<4.0, iOS<6 (functionish RegExp)
 return (typeof obj === "object" || typeof obj === "function") ?
  (class2type[ toString.call(obj) ] || "object") :
  typeof obj;
},

// 省略部分代码... 

// Populate the class2type map
jQuery.each("Boolean Number String Function Array Date RegExp Object Error".split(" "), function(i, name) {
 class2type[ "[object " + name + "]" ] = name.toLowerCase();
});

// Populate the class2type map
jQuery.each("Boolean Number String Function Array Date RegExp Object Error".split(" "), function(i, name) {
 class2type[ "[object " + name + "]" ] = name.toLowerCase();
});

//循环之后，`class2type`的值是： 
class2type = {
 '[object Boolean]' : 'boolean', 
 '[object Number]' : 'number',
 '[object String]' : 'string',
 '[object Function]': 'function',
 '[object Array]'  : 'array',
 '[object Date]'  : 'date',
 '[object RegExp]' : 'regExp',
 '[object Object]' : 'object',
 '[object Error]'  : 'error'
}

// type的实现
type: function( obj ) {
 // 若传入的是null或undefined，则直接返回这个对象的字符串
 // 即若传入的对象obj是undefined，则返回"undefined"
 if ( obj == null ) {
  return obj + "";
 }
 // Support: Android<4.0, iOS<6 (functionish RegExp)
 // 低版本regExp返回function类型；高版本已修正，返回object类型
 // 若使用typeof检测出的obj类型是object或function，则返回class2type的值，否则返回typeof检测的类型
 return (typeof obj === "object" || typeof obj === "function") ?
  (class2type[ toString.call(obj) ] || "object") :
  typeof obj;
}