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;

console.log(aa.hasOwnProperty("xx"));

Array also has the attribute β€œextensible”, 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

Array.isArray

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 β€œlength” 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

String.prototype.split

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

console.log(
 Object.getOwnPropertyNames(hh)
); // [ '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

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