Detailed introduction to JS judgment array

How to determine the array? Some friends should know, but some can't tell how js judges arrays. The editor below has collected and sorted out some information for you on this issue. Friends who are interested can take a look at it. I hope you can master the knowledge of JS to judge arrays.

typeof operator

typeof will return a string of this type

 var a = '123'
 console.log(typeof(a)) //string
 var b = []
 console.log(typeof(b)) //object
 var c = {}
 console.log(typeof(c)) //object
 var d = null
 console.log(typeof(d)) //object

As seen above, the array object null is returned by typeof. This method cannot identify whether it is an object. Array

Prototype contructor chain method

Instantization has a contructor attribute. This attribute points to the method of generating an object array

 var a = []
 console.log(a.__proto__.constructor) //ƒ Array() { [native code] }
 var b = {}
 console.log(b.__proto__.constructor) //ƒ Object() { [native code] }

As seen above, the array is an object instantiated by the Array function. Instantiated by the Object function

I feel that this method is OK, but the constructor property can be overwritten

  var a = []
  a.__proto__.constructor = Object
  console.log(a.__proto__.constructor) //ƒ Object() { [native code] }

You can see that this has become an array and judged to be an object, so this The method is not the best either

instanceof

This method is to determine whether the object pointed to by the prototype attribute of a certain constructor is on the prototype chain of another object to be detected

  var a = []
  console.log(a instanceof Array) //a对象的原型链上能找到Array true
  console.log(a instanceof Object) //true 原型链上也能找到对象

The above is not particularly easy to determine whether it is an array or an object

General method toString

toString() method returns the string showing this object

  var a= '123'
  console.log(a.toString()) //123
  var b = [1,2,3]
  console.log(b.toSting()) //1,2,3
  var c = {}
  console.log(c.toString)) //[object Object]

You can see that only the object returns the object type

Returns [object type] type represents the type of the object

To determine the object, use the toString method of Object to take it and use

 var a =[]
 Object.prototype.toString.call(a) //[object Array]

this object The toString method can determine whether it is an array

But please note that there is a situation where toString() on the object prototype can also be changed

Array.isArray(XX)

My personal feeling is to use the general method toString() method Reliable

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