What is the reason behind the length property belonging to an array object?

While there are other instances, let's focus on the length property for now. Why does it appear as if we're copying it here:

[].hasOwnProperty("length")
//==> true

It is common knowledge that an array's length property belongs to Array.prototype, making it easily accessible from any array instance through the prototype chain. So why the apparent duplication? Could this be related to a specific browser implementation? (The code example above was tested in the Chrome console). And even according to MDN: "...methods and properties are not copied from one object to another in the prototype chain. They are accessed by walking up the chain..."

javascriptinheritanceprototype-chain

Answer №1

The reason for this is quite simple: each array instance has its own unique .length value.

In an alternative scenario, the .length property could have functioned as a getter/setter inherited from a shared prototype object. However, the original design of JavaScript opted for a data property that updates automatically with element creation or removal.

The length property is located on Array.prototype

While there may be an Array.prototype.length property, it only exists because Array.prototype is an array itself. It's worth noting that the value is 0, which wouldn't be ideal to inherit across all arrays.

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