日常开发小汇总（3）js类型判断

1.typeof 能判断出字符串、数字、方法和undefined，array、null、object判断不出

        let num = 1;
        let str = "x";
        let fn = function user(){}
        let arr = [1,2]
        let obj = {name:"zhangs"}
        let und;
        let nul = null;
        console.log(typeof num) //number
        console.log(typeof str) //string
        console.log(typeof fn)//function
        console.log(typeof und) //undefined
        console.log(typeof arr) //object
        console.log(typeof obj) //object
        console.log(typeof nul) //object
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2. instanceof 如 a instanceof b, 判断a的原型链上是否有b的原型

        let fn = function user(){}
        let arr = [1,2]
        let obj = {name:"zhangs"}
        let und;
        console.log(fn instanceof Function) //true
        console.log(arr instanceof Array) //true
        console.log(arr instanceof Object) //true
        console.log(obj instanceof Object) //true
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3. Object.getPrototypeOf() 返回参数的隐式原型 ，感觉有些鸡肋
   Object.getPrototypeOf ([]) === Array.prototype //true

4. Object.prototype.toString.call()
   这里解释一下为什么Object.prototype.toString 可以调用call方法，首先我们知道call方法来源，它来自Function.prototype,
   
   这是因为Object.prototype.toString 本身是一个方法，是方法它的原型链上就会出现Function的原型对象，所有就有了call方法，这跟一个对象如let obj= {}，obj可以使用toString一个意思，因为toString在Object.prototype上。

let num = 1;
        let str = "x";
        let fn = function user(){}
        let arr = [1,2]
        let obj = {name:"zhangs"}
        let und;
        let nul = null;
        console.log(Object.prototype.toString.call(obj)); //[object Object]
        console.log(Object.prototype.toString.call(fn)); //[object Function]
        console.log(Object.prototype.toString.call(arr)); //[object Array]
        console.log(Object.prototype.toString.call(nul)); //[object Null]
        console.log(Object.prototype.toString.call(und)); //[object Undefined]
        console.log(Object.prototype.toString.call(num)); //[object Number]
        console.log(Object.prototype.toString.call(str)); //[object String]
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这种方式还是挺万能的，没看出来有什么局限性