6 ways to determine whether it is an array in JS

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1. Array.isArray judgment

Usage: Array.isArray(arr)

The Array.isArray method is added in ES5, IE8 and The following is not supported

Array.isArray() Used to determine whether the passed value is an [Array], returning a Boolean value of true; otherwise it returns false.

let arr = [];
console.log(Array.isArray(arr)); // true

// 下面的函数调用都返回 true
Array.isArray([]);
Array.isArray([1]);
Array.isArray(new Array());
Array.isArray(new Array('a', 'b', 'c', 'd'))
// 鲜为人知的事实：其实 Array.prototype 也是一个数组。
Array.isArray(Array.prototype);

2. Constructor judgment

##Usage: arr.constructor === Array

constructor, which is used to save the function used to create the current object

let arr = [];
console.log(arr.constructor === Array); // true

3. instanceof judgment

Usage: arr instanceof Array

instanceof is mainly used to determine an instance Whether it belongs to an object

let arr = [];
console.log(arr instanceof Array); // true

4. Judgment of isPrototypeOf on the prototype chain

Usage: Array.prototype.isPrototypeOf(arr)

Array.prototype The attribute represents the prototype of the Array constructor

isPrototypeOf()Can be used to test whether an object exists on the prototype chain of another object.

let arr = [];
console.log(Array.prototype.isPrototypeOf(arr)); // true

5. Object.prototype.toString

Object.prototype.toString.call (arr) === '[object Array]'Array inherits from Object, JavaScript overrides toString on

, toString.call( arr) is actually called through the prototype chain.

let arr = [];
console.log(Object.prototype.toString.call(arr) === '[object Array]'); // true

6. isPrototypeOf on the Array prototype chain

Array.prototype.isPrototypeOf( arr)

The attribute represents the prototype of the Array constructor

let arr = [];
console.log(Array.prototype.isPrototypeOf(arr)); // true

By the way, review the usage of typeof: For reference types, typeof cannot be used to judge, because all returned are objects

// 基本类型
typeof 123;  //number
typeof "abc"; //string
typeof true; //boolean
typeof undefined; //undefined
typeof null; //object
var s = Symbol;
typeof s; //symbol

// 引用类型
typeof [1,2,3]; //object
typeof {}; //object
typeof function(){}; //function
typeof  Array; //function  Array类型的构造函数
typeof Object; //function  Object类型的构造函数
typeof Symbol; //function  Symbol类型的构造函数
typeof Number; //function  Number类型的构造函数
typeof String; //function  String类型的构造函数
typeof Boolean; //function  Boolean类型的构造函数

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