aaa.数据类型判断

1.用typeof操作符判断基本数据类型和function类型

"undefined"——未定义
"boolean"——布尔
"string"——字符串
"number"——数值
"object"——对象或者null
"function"——函数

var param1 = false
var param2 = 'fdsafdsa'
var param3 = 123
var param4 = {}
var param5 = null
var param6 = function () {return 1}
var param7
console.log(typeof(param1),typeof(param2),typeof(param3))
console.log(typeof(param4),typeof(param5),typeof(param6),typeof(param7))
//boolean string number
//object object function undefined

此操作符可以判断除了object & null以外的所有类型（null是空对象指针）

2.判断是否为数组类型

主要就是数组对象和null不好判断

1. instanceof
   a instanceof Array检测Array的prototype是否在a的原型链上

var param1 = ['12', '23231', 123]
var param2 = { a: '123', b: '111' }
var param3 = null
var result = function (par) {
  return (par instanceof Array)
}
console.log(result(param1))       //true
console.log(result(param2))       //false
console.log(result(param3))       //false

只有数组在这种情况下是true，是唯一，可以用于判断

2. 原型链方法constructor

var param1 = ['12', '23231', 123]
var param2 = { a: '123', b: '111' }
var param3 = null
var param4
var result = function (par) {
  if(par){
  return (par.constructor == Array)
}else{
  return 'result is null'
}
}
console.log(result(param1))      //true
console.log(result(param2))      //false
console.log(result(param3))      //result is null
console.log(result(param4))      //result is null

3. Object.prototype.toString.call

var param1 = ['12', '23231', 123]
var param2 = { a: '123', b: '111' }
var param3 = null
var result = function (par) {
 return Object.prototype.toString.call(par);
}
console.log(result(param1))          //[object Array]
console.log(result(param2))          //[object Object]
console.log(result(param3))          //[object Null]

Object.prototype.toString.call(o)=='[object Array]';用这个判断

4. isArray

var param1 = ['12', '23231', 123]
var param2 = { a: '123', b: '111' }
var param3 = null
var result = function (par) {
 return Array.isArray(par);
}
console.log(result(param1))       //true
console.log(result(param2))       //false
console.log(result(param3))       //false

3.判断是否为对象

1. constructor

var param1 = ['12', '23231', 123]
var param2 = { a: '123', b: '111' }
var param3 = null
var param4 = function(){ return 1}
var param5 = new param4()
var result = function (par) {
  if(par){
    return par.constructor
  }else{
    return 'this is null'
  }
}

console.log(result(param4))        //[Function: Function]
console.log(result(param1))        //[Function: Array]
console.log(result(param2))        //[Function: Object]
console.log(result(param3))        //this is null
console.log(result(param5))        //[Function: param4]

constructor可以用于判断array,object,null三种数据类型

2. Object.prototype.toString.call()
   最好用的方法，最推荐

var param1 = ['12', '23231', 123]
var param2 = { a: '123', b: '111' }
var param3 = null
var param4 = function(){ return 1}
var param5 = new param4()
var result = function (par) {
return Object.prototype.toString.call(par)
}
console.log(result(param1))        //[object Array]
console.log(result(param2))        //[object Object]
console.log(result(param3))        //[object Null]
console.log(result(param4))        //[object Function]
console.log(result(param5))        //[object Object]

3. 比较constructor和Objcet.prototype.toString区别

function cla(){}
function cla2(){}
function cla3(){}
cla.prototype = new cla2();
var a = new cla();
var b = new cla3();
console.log(a.constructor);           //[Function: cla2]
console.log(b.constructor)          //[Function: cla3]
console.log(Object.prototype.toString.call(a))        //[object Object]
console.log(Object.prototype.toString.call(b))        //[object Object]

ps:constructor检测的是对象prototype上的类型，所以在new一个方法做对象的时候，检测出来的不会return objcet，会返回function方法名，而且可以修改par.prototype = new newClass()会被连带修改类型，导致检测结果不准确，所以不推荐使用constructor，更推荐万能的Objcet.prototype.toString.Call()