var name = "Jack";
var age = 32;
var single = false;
var app;  //undefined

console.log(typeof name);  //string
console.log(typeof age);  //number
console.log(typeof single); //boolean
console.log(typeof app);  //undefined
console.log(typeof null);  //object

if(typeof name === "string") { name += "Zhang"; }
if(typeof age === "number") { age++; }
if(typeof single === "boolean" && single) { … }
if(typeof app === "undefined") { app = {}; }

if(el === null) { … }

console.log(typeof new Date());  //object
console.log(typeof new RegExp()); //object
console.log(typeof new Error());  //object
console.log(typeof new Person()); //用typeof检测出自定义对象也是object

var date = new Date();
var reg = new RegExp();
var err = new Error();
var me = new Person();

if(date instanceof Date) {  //检测日期
  year = date.getFullYear(); 
}
if(reg instanceof RegExp) {  //检测正则表达式
  reg.test(...); 
}
if(err instanceof Error) {  //检测异常
  throw err; 
}
if(me instanceof Person) {  //检测自定义对象
  ... 
}

var func = function(){};
if(typeof func === 'function') { … }

console.log(typeof document.getElementById);    //object，不是function
console.log(typeof document.getElementsByTagName); //object，不是function
console.log(typeof document.createElement);     //object，不是function

//IE8以前的IE浏览器，要改用in来检测是否支持DOM函数
if("getElementById" in document) { … }    
if("getElementsByTagName" in document) { … }
if("createElement" in document) { … }

function isArray(arr) {
  return Object.prototype.toString.call(arr) === "[Object Array]";
}

function isArray(arr) {
  return typeof arr.sort === "function";
}

var Person = {
  name: "Jack",
  age: 33
};
if("name" in Person) { … }         //true
if(Person.hasOwnProperty("name")) { … }  //true

var Person = function (name, age) {
  this.name = name;
  this.age = age;
};
Person.prototype.location = "Shanghai";

var me = new Person("Jack", 33)
if("name" in me) { … }         //true
if(me.hasOwnProperty("name")) { … }  //true
if("location" in me) { … }       //true
if(me.hasOwnProperty("location")) { … }//false

if (object[propName])      //Not Good，你怎么知道属性值不是0或1？
if (object[propName] === null)    //Not Good，你怎么知道属性值不是null？
if (object[propName] === undefined)  //Not Good，你怎么知道属性值不是undefined？