Judge whether the variable is an array

Hello everyone , I'm brother watermelon , Today I will take you to learn in JS How to determine whether an object is an array .

Array.isArray

The best way to write it is to use Array.isArray(val).

Because this method can correctly judge iframe The variable passed .

instanceof

val instanceof Array

For a normal array , Its prototype chain is like this ：

 Array instance  -> Array.protype -> Object.prototype -> null

If a variable is an array , Then there must be on its prototype chain Array.prototype.

instanceof In fact, the principle of l-value follows its prototype chain , And right value prototype Attribute comparison , See if it's equal . Find the equivalent and return true, Otherwise return to false.

but instanceof Unable to deal with iframe The variable of , because iframes Of Array Not the current page Array, They point to their respective memory space .

const iframe = document.createElement('iframe');
document.body.appendChild(iframe);

const ArrayInFrame = window.frames[0].Array; // iframe Constructors in 
const arr = new ArrayInFrame();

arr instanceof Array // false
Array.isArray(arr); // true

As long as you can judge Whether the object is on the prototype chain All methods can be used to determine whether it is an array , So you can also use ：

// isPrototypeOf  Method 
// Array.prototype  Whether in  val  On the prototype chain 
Array.prototype.isPrototypeOf(val);

// getPrototypeOf  Method 
// val  Whether the last prototype of is  Array.prototype
Object.getPrototypeOf(val) === Array.prototype

Object.prototype.toString.call

Object.prototype.toString.call(val) === '[object Array]';

Objects can be accessed through toString Method to generate a string , For the object , Usually back to [object Xxx] A string in the form of , For arrays , Will return [object Array].

ending

Considering compatibility iframe The variable of , So it is recommended to use Array.isArray programme .

In addition, the method of finding the prototype chain object can also be used to determine the array , And by toString The method of getting the string .

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