//判断变量num是不是一个数字类型
if(typeof num=='number') {
  return true;
}

var a="hling"; console.log(a); //string
var a=1; console.log(a); //number
var a=false; console.log(a); //boolean
var a; console.log(typeof a); //undfined

var a = null; console.log(typeof a); //object
var a = document; console.log(typeof a); //object
var a = []; console.log(a); //object

var a = function(){}; console.log(typeof a) 
//function除了可以判断数据类型还可以判断function类型

a instanceof b?alert("true"):alert("false") 
//注意b值是你想要判断的那种数据类型，是不是一个字符串，比如Array

var arr = [1,2,3,1]; 
alert(arr instanceof Array); // true 

var arr = []; 
arr instanceof Array; // true 
arr.constructor == Array; //true

console.log("string".constructor == String);
console.log((123).constructor == Number);
console.log(false.constructor == Boolean);
console.log([].constructor == Array);
console.log({}.constructor == Object);

function isArray(object){
  return object && typeof object==='object' &&
      Array == object.constructor;
}

function isArray(object){
  return object && typeof object==='object' &&  
      typeof object.length==='number' && 
      typeof object.splice==='function' &&  
       //判断length属性是否是可枚举的 对于数组 将得到false 
      !(object.propertyIsEnumerable('length'));
}

Object.prototype.toString.call(value) == '[object Array]'