判断数据类型的方法？instanceof原理?判断空对象？ typeof null？typeof NaN？

判断数据类型的方法

typeof

判断数据类型，它返回表示数据类型的字符串（返回结果只能包括number,boolean,string,function,object,undefined）

typeof判断变量是否存在（如if(typeof a!="undefined"){...}）

typeof null
"object"

typeof undefined
"undefined"

typeof NaN
"number"

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instanceof

A instanceof B 可以判断A是不是B的实例，返回一个布尔值，由构造类型判断出数据类型

[1,2,3] instanceof Array
true

function Car(make, model, year) {
  this.make = make;
  this.model = model;
  this.year = year;
}
const auto = new Car('Honda', 'Accord', 1998);

console.log(auto instanceof Car);
// expected output: true

console.log(auto instanceof Object);
// expected output: true

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Object下的toString.call()

Object.prototype.toString.call([])
"[object Array]"
Object.prototype.toString.call({})
"[object Object]"
Object.prototype.toString.call(function(){})
"[object Function]"

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 contructor

[].constructor
ƒ Array() { [native code] }

"123".constructor
ƒ String() { [native code] }

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判断空对象

JSON.stringify()

var obj = {}
var obj1 = new Object()
JSON.stringify(obj)
"{}"
JSON.stringify(obj1)
"{}"

Object.keys

let obj3 = {}
Object.keys(obj3).length
0

Object.getOwnPropertyNames

Object.getOwnPropertyNames(obj3)
[]

obj3 = {a:1,
   [Symbol('name')]: 'yr'}
{a: 1, Symbol(name): "yr"}
Object.getOwnPropertyNames(obj3)
["a"]

//判断length为0则为空对象

for...in

function isEmptyObject(obj){
    for(var key in obj){
        return false
    };
    return true
};

var a1={};
isEmptyObject(a1)
true

var b1 = {a:2}
isEmptyObject(b1)
false