Four methods for judging JS data types and user-defined methods

Preface

stay ECMAScript Specification , Altogether 7 Data type in , It is divided into basic types and reference types （ Complex type ）,

• Basic types ：String,Number,Boolean,Symbol,Null,Undefined
• Reference type ：Object

Method 1 ,typeof()

Use typeof Directly return the data type field , But it's impossible to judge Null, Array , object

console.log(typeof bool); //boolean
console.log(typeof num);//number
console.log(typeof str);//string
console.log(typeof und);//undefined
console.log(typeof nul);//object
console.log(typeof arr);//object
console.log(typeof obj);//object
console.log(typeof fun);//function

It can be seen that , about Null, Array , object ,typeof The test results are object.
For function types , return Function

Method 2 ,instance of()

instance of Used to determine A Is it B Example , It is impossible to determine which type an object instance belongs to .
instance of What we tested was Prototype

console.log(bool instanceof Boolean);// false
console.log(num instanceof Number);// false
console.log(str instanceof String);// false
console.log(und instanceof Object);// false
console.log(arr instanceof Array);// true
console.log(nul instanceof Object);// false
console.log(obj instanceof Object);// true
console.log(fun instanceof Function);// true

var bool2 = new Boolean()
console.log(bool2 instanceof Boolean);// true

var num2 = new Number()
console.log(num2 instanceof Number);// true

var str2 = new String()
console.log(str2 instanceof String);// true

function Person(){
    }
var per = new Person()
console.log(per instanceof Person);// true

function Student(){
    }
Student.prototype = new Person()
var haoxl = new Student()
console.log(haoxl instanceof Student);// true
console.log(haoxl instanceof Person);// true

instance of There is no difference Null and Undefined

Method 3 ,constructor

undefined and null No, contructor attribute

console.log(bool.constructor === Boolean);// true
console.log(num.constructor === Number);// true
console.log(str.constructor === String);// true
console.log(arr.constructor === Array);// true
console.log(obj.constructor === Object);// true
console.log(fun.constructor === Function);// true

console.log(haoxl.constructor === Student);// false
console.log(haoxl.constructor === Person);// true

Method four ,toString()

toString() yes Object Prototype method of , Call the method , Returns the current object's [[Class]] . This is an internal property , The format for [object Xxx] , among Xxx Is the type of object .

about Object object , Call directly toString() Can return [object Object] . For other objects , You have to go through call / apply To return the correct type information .

call() The method can change this The direction of , Then put the Object.prototype.toString() Methods point to different data types , Return different results

Object.prototype.toString.call(1)
"[object Number]"

Object.prototype.toString.call(NaN);
"[object Number]"

Object.prototype.toString.call("1");
"[object String]"

Object.prototype.toString.call(true)
"[object Boolean]"

Object.prototype.toString.call(null)
"[object Null]"

Object.prototype.toString.call(undefined)
"[object Undefined]"

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

Object.prototype.toString.call([]);
"[object Array]"

Object.prototype.toString.call({
    });
"[object Object]"

Customize a perfect method to judge the data type

Reference documents ： javascript Several ways to determine the type of data

function _typeof(obj){
    
  var s = Object.prototype.toString.call(obj);
  return s.match(/\[object (.*?)\]/)[1].toLowerCase();
};


_typeof([12,3,343]);
"array"

_typeof({
    name: 'zxc', age: 18});
"object"

_typeof(1);
"number"

_typeof("1");
"string"

 _typeof(null);
"null"

_typeof(undefined);
"undefined"

_typeof(NaN);
"number"

_typeof(Date);
"function"

_typeof(new Date());
"date"

_typeof(new RegExp());
"regexp"