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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