JavaScript: Array Overview

By Xah Lee. Date: . Last updated: .

JavaScript array is a Object, with a magic property key "length", and special treatement of non-negative integer string property keys "0", "1", "2", etc.

Array Syntax

Array Basics

Array is Object Type with Special Purpose

JavaScript Array is a Object. Meaning, it is a key/value pairs. For example, we can add property to array that's not an integer:

typeof [3,4] === "object"

// array is a object, you can add properties to it
const aa = [3, 4];
aa.xx = 7;


Array also has the attribute “extensible”, just like other objects. [see Prevent Adding Property]

Object.isExtensible( [3,4] )

Index vs Property Key

The index of array is the same as string property key.

[3, 4].hasOwnProperty("0")

Non-Existent Index Return Undefined

// accessing array with non-existent index results undefined
const arr = [3];
console.log(arr[200] === undefined);

Check If a Object is Array


Length Special Property

Every array object has a special property key "length". It is the object's own property, and is a key of type string. The “length” property is automatically updated when array elements are added or removed using Array.prototype methods.

Array length can be set. If you set it, the array will be lengthened or shortened.

// creating a sparse array by setting the length property

const aa = ["a", "b"];

// set the length property beyond the last index
aa.length = 3;

console.log(aa.length); // 3

console.log( Object.getOwnPropertyNames(aa)); // [ '0', '1', 'length' ]

console.log(aa); // [ 'a', 'b',  ]
// truncating a array by setting its length

const aa = ["a", "b", "c"];
console.log(aa.length); // 3

aa.length = 1;

console.log(aa.length); // 1
console.log(aa[1]);     // undefined

console.log( Object.getOwnPropertyNames(aa)); // [ '0', 'length' ]

[see Array.prototype.length]

Array Methods

Common array operations should be done using array methods, such as shift, unshift, push, pop. [see Array.prototype]

To add/remove element(s) in middle, use Array.prototype.splice.

Warning: Never use “delete” Operator to remove element in a array, because that creates a Sparse Array.

String to Array


Max Number of Elements

The max number of elements is 2^32 - 1 (which is 4 294 967 295).

When the index is a number between 0 to 2^32-2, inclusive, it is treated as array index, else it's just a property key.

// When array index is beyond 2^32 - 2, it is treated as a property key.

const hh = ["a"];
hh[1] = "b";
hh[2**32] = "c";

console.log( 2**32 === 4294967296 );

console.log(hh); // [ 'a', 'b', '4294967296': 'c' ]
// note, the c is printed differently

); // [ '0', '1', 'length', '4294967296' ]
// all are property keys

Note: if you create a array with just 1,000,000 items, the browser will become not responsive.

Array is Iterable

That means, you can use for-of Loop and Spread Operator on them.

JavaScript Array

JavaScript in Depth