js判斷資料型別

typeof可以判斷出基礎資料型別以及object、function，區分不出object的具體型別

let a

typeof a //undefined
a =123typeof a //number
a ='124'
typeof a //string
a =null
typeof a //object
a =typeof a //objecta=
function()
typeof a //function
a =symbol
('id');
typeof a //symbol
a =123n
typeof a //bigint
a =newdate()
typeof a //object

instanceof原理:左邊是乙個例項物件，右邊是乙個建構函式，instanceof會檢查建構函式的原型物件prototype是否在左邊物件的原型鏈上，有則返回true,否則返回false.

instanceof(a

,b)=
return
false
;}

let a =

a instanceof
object
//true
a =[1
,2] a instanceof
array
//true
a =newdate()
a instanceof
date
//true
function
mytest()
a =newmytest()
a instanceof
mytest
//true
a instanceof
object
//true

let a =

'1234'
a.constructor === string //true
a =1234
a.constructor === number //true
a =a.constructor === object //true
a =[1
,2] a.constructor === array //true
a =newdate()
a.constructor === date //true
a =1234n
a.constructor === bigint //true
a =symbol
('id'
) a.constructor === symbol //true
a =true
a.constructor === boolean //true
function
mytest()
a =newmytest()
a.constructor === mytest //true

let a

object.prototype.tostring.
call
(a)//[object undefined]
a =123 object.prototype.tostring.
call
(a)//[object number]
a ='124'
object.prototype.tostring.
call
(a)//[object string]
a =null
object.prototype.tostring.
call
(a)//[object null]
a =object.prototype.tostring.
call
(a)//[object object]a=
function()
object.prototype.tostring.
call
(a)//[object function]
a =symbol
('id');
object.prototype.tostring.
call
(a)//[object symbol]
a =123n 
object.prototype.tostring.
call
(a)//[object bigint]
a =newdate()
object.prototype.tostring.
call
(a)//[object date]

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