为什么用{}.toString.call(obj)来判断数据类型

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先看一下typeof

var a;console.log("a:"+typeof a)//a:undefined var b=null;console.log("b:"+typeof  b)//b:object var c=undefined;console.log("c:"+typeof c)//c:undefined var d=new Object();console.log("d:"+typeof d)//d:object var e=function(){};console.log("e:"+typeof e)//e:function var f={};console.log("f:"+typeof f)//f:object var g="";console.log("g:"+typeof g)//g:string var h=[];console.log("h:"+typeof h)//h:object var i=true;console.log("i:"+typeof i)//i:boolean var j=123 console.log("j:"+typeof j)//j:number var k=NaN;console.log("k:"+typeof  k)//k:number var l=/^[-+]?\d+$/;console.log("l:"+typeof  l)//l:object

按照上面的打印结果，总结出以下几点：

1 除了函数 function，typeof ‘引用类型’ 都是object 例如type [];typeof /^\d$/ ;typeof null

2 typeof null 为object ,typeof NaN 是number,typeof undefined是undefined,typeof function(){}是function

3 转换数字的常见用法“10”-0，如果没有转换成功则返回NaN,由于NaN的一个特性NaN！=NaN

故判断转换成功与否的常见做法是：('10sss'-0)==('10sss'-0)//结果为假 *******

再看看constructor和instanceof

instanceof 用于判断一个对象是否是某个类的实例

constructor 用于判断对象的原型，该属性返回对创建此对象的数组函数的引用。

   console.log("-------------------Number----------------")    var A=123;   console.log(A instanceof Number)//false    console.log(A.constructor===Number)//true    console.log(A.constructor)    console.log("-----------------String------------------")    var B="javascript";   console.log(B instanceof String)//false    console.log(B.constructor===String)//true    console.log(B.constructor)   console.log("-----------------Boolean------------------")    var C=true;   console.log(C instanceof Boolean)//false    console.log(C.constructor===Boolean)//true    console.log(C.constructor)    console.log("------------------null------------------------")   var D=null    console.log(D instanceof Object)//false//    console.log(D.constructor===Object)//报错 null没有构造函数   //console.log(D.constructor)//报错console.log("----------------------undefined---------------------")    var E=undefined;//   console.log(E instanceof undefined)//报错 没有undefined数据类    console.log("---------------------function---------------------")    var F=function(){}    console.log(F instanceof Function)//true    console.log(F.constructor=== Function)//true    console.log(F.constructor)console.log("---------------------new function----------------------")    function SB(){}    var G=new SB();    console.log(G instanceof SB)//true    console.log(G.constructor==SB)//true    console.log(G.constructor)console.log("----------------------new Object------------------------")    var H=new Object;    console.log(H instanceof Object)//true    console.log(H.constructor==Object)//true    console.log(H.constructor)    console.log("---------------------Array---------------------------")    var I=[];    var H=new Array();    console.log(I instanceof Array);//true    console.log(H instanceof Array);//true    console.log(I.constructor==Array)//true    console.log(H.constructor==Array)//true    console.log(H.constructor)//true    console.log(I.constructor)//true    console.log("--------------------------------JSON---------------")    var J={        "sb":"javascript",        "node":"very SB"    }    console.log(J instanceof Object)//true    console.log(J.constructor===Object)//true    console.log(J.constructort)//true

再看看{}.toString.call(obj)

console.log({}.toString.call(1))//[object Number]console.log({}.toString.call("11"))//[object String]console.log({}.toString.call(/123/))//[object RegExp]console.log({}.toString.call({}))//[object Object]console.log({}.toString.call(function(){}))//[object Function]console.log({}.toString.call([]))//[object Array]console.log({}.toString.call(true))//[object Boolean]console.log({}.toString.call(new Date()))//[object Date]console.log({}.toString.call(new Error()))//[object Error]console.log({}.toString.call(null))//[object Null]console.log({}.toString.call(undefined))//[object Undefined]console.log(String(undefined))//undefinedconsole.log(String(null))//null

参照：https://www.talkingcoder.com/article/6333557442705696719

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