Why an array is an object in JavaScript?

JS is a prototype-based language, so there are only primitive types and objects. It grants flexibility but makes things more confusing at the same time.

👉 Everything is an object!

Implementation of all non-primitive values in JavaScript is object-based.
Simply put, JavaScript has a single prototypical object from which all other objects get their initial properties. We can get it by accessing __proto__.

Object.getPrototypeOf(Object).__proto__;
Object.getPrototypeOf(Array).__proto__;
Object.getPrototypeOf(Boolean).__proto__;

// The prototypical object of every object
{
  constructor: ƒ Object()
  hasOwnProperty: ƒ hasOwnProperty()
  isPrototypeOf: ƒ isPrototypeOf()
  propertyIsEnumerable: ƒ propertyIsEnumerable()
  toLocaleString: ƒ toLocaleString()
  toString: ƒ toString()
  valueOf: ƒ valueOf()
  __defineGetter__: ƒ __defineGetter__()
  __defineSetter__: ƒ __defineSetter__()
  __lookupGetter__: ƒ __lookupGetter__()
  __lookupSetter__: ƒ __lookupSetter__()
  __proto__: (...)
  get __proto__: ƒ __proto__()
  set __proto__: ƒ __proto__()
}

👉 Every array is an object too!

The array type is not an exception here. Array global class is a global object and an array literal is just an instance of the Array global class.
In turn, a direct prototype of the array type contains all its special methods, like fill, find, etc.

// true
Object.getPrototypeOf(Array).__proto__ === Object.getPrototypeOf(Object).__proto__
Object.getPrototypeOf([]).__proto__ === Object.getPrototypeOf(Object).__proto__


Object.getPrototypeOf([])
[
  at:  ƒ at()
  concat:  ƒ concat()
  constructor:  ƒ Array()
  copyWithin:  ƒ copyWithin()
  entries:  ƒ entries()
  every:  ƒ every()
  fill:  ƒ fill()
  filter:  ƒ filter()
  find:  ƒ find()
  findIndex:  ƒ findIndex()
  findLast:  ƒ findLast()
  findLastIndex:  ƒ findLastIndex()
  flat:  ƒ flat()
  ...
]

👉 How is it implemented in the JavaScript engine?

Similarly, arrays are a special case of objects inside the JavaScript engine.
But they have:

• special handling of indices
• a magical length property

To understand how objects work, check out my article.

👉 Indices handling

Array indices are represented as strings, that contain numbers.
So every element inside an array is associated with a numeric string.

👉 Length property

The length is just non-configurable and non-enumerable property. The JavaScript engine automatically updates its value once an element is added to the array or deleted from it.

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