JS中数据类型检测的方式

1、typeof

console.log(typeof 2);            //number
console.log(typeof true);         //boolean
console.log(typeof 'string');     //string
console.log(typeof []);           //object
console.log(typeof function(){
    
    }); //function
console.log(typeof {
    
    });           //object
console.log(typeof undefined);    //undefined
console.log(typeof null);         //object

2、instanceof

  instanceof 可以判断对象的类型，其内部运行机制是判断原型链中能否找到该类型的原型
  console.log(2 instanceof Number);               //false
  console.log(true instanceof Boolean);           //false
  console.log('str' instanceof String);           //false
  console.log([] instanceof Array);               //true
  console.log(function(){
    
    } instanceof Function);  //true
  console.log({
    
    } instanceof Object);              //true

3、constructor

  constructor有两个作用，一是判断数据的类型，二是对象实例通过constructor对象访问它的构造函数
  console.log((2).constructor === Number);               //true
  console.log((true).constructor === Boolean);           //true
  console.log(('str').constructor === String);           //true
  console.log(([]).constructor === Array);               //true
  console.log((function(){
    
    }).constructor === Function);  //true
  console.log(({
    
    }).constructor === Object);              //true

4、Object.prototype.toString.call()

var a = Object.prototype.toString;
console.log(a.call(2));           //[object Number]
console.log(a.call(true));        //[object Boolean]
console.log(a.call('string'));    //[object String]
console.log(a.call([]));          //[object Array]
console.log(a.call(function(){
    
    }));//[object Function]
console.log(a.call({
    
    }));          //[object Object]
console.log(a.call(undefined));   //[object undefined]
console.log(a.call(null));        //[object null]