Unusual Array Artifacts: Exploring JavaScript Arrays

Question

Unusual Array Artifacts: Exploring JavaScript Arrays

When it comes to JavaScript, arrays have their own unique quirks that set them apart from other programming languages.

Take this example:

// Creating a regular array.
var regularArray = [];
regularArray.length = 0;

regularArray.push(1);
regularArray[1] = 2;

regularArray; // returns [1, 2]
regularArray.length // returns 2

We all know how to create and populate arrays like the above, right? (let's ignore the normalArray.length = 0 for now)

But what happens when we try the same process on an object that is not purely an array? The outcome looks slightly different, with the length property being somewhat inaccurate.

// Creating an object that inherits from the array prototype (i.e.: custom array)
var customArray = new (function MyArray() {
    this.__proto__ = Object.create(Array.prototype);
    return this
});
customArray.length = 0;

customArray.push(1);
customArray[1] = 2;

customArray; // returns [1, 1: 2]
customArray.length // returns 1

The behavior here is puzzling, so any help in understanding it would be greatly appreciated.

javascript

Answer 1

Answer №1

In the world of Javascript arrays, there exists a nuanced difference between arrays and regular objects. One key distinction is that arrays have a length property while objects do not.

For example, creating a standard array looks like this:

var normalArray = [];

But what about this alternative method?

var customArray = new (function MyArray() {
    this.__proto__ = Object.create(Array.prototype);
    return this
});

Are they truly equivalent? Let's investigate:

Array.isArray(normalArray); // true -> [object Array]
Array.isArray(customArray); // false -> [object Object]

It becomes apparent that although we are inheriting from the array prototype, the result is a plain JavaScript object with inherited array functions rather than an actual Array type. This explains why updating the value in customArray.push(1); affects the length accordingly.

However, as customArray is fundamentally just a regular object, using the notation [] to set a property does not impact the length property (as objects lack a length property).

Hopefully, this explanation provides clarity on the distinctions between arrays and regular objects in Javascript :)

Answer 2

In the world of Javascript arrays, there exists a nuanced difference between arrays and regular objects. One key distinction is that arrays have a length property while objects do not.

For example, creating a standard array looks like this:

var normalArray = [];

But what about this alternative method?

var customArray = new (function MyArray() {
    this.__proto__ = Object.create(Array.prototype);
    return this
});

Are they truly equivalent? Let's investigate:

Array.isArray(normalArray); // true -> [object Array]
Array.isArray(customArray); // false -> [object Object]

It becomes apparent that although we are inheriting from the array prototype, the result is a plain JavaScript object with inherited array functions rather than an actual Array type. This explains why updating the value in customArray.push(1); affects the length accordingly.

However, as customArray is fundamentally just a regular object, using the notation [] to set a property does not impact the length property (as objects lack a length property).

Hopefully, this explanation provides clarity on the distinctions between arrays and regular objects in Javascript :)

Answer 3

Answer №2

The structure you are attempting to construct is not a traditional array, but rather a JavaScript object that is meant to function like one.

When treating an object as an array, it is your responsibility to uphold all of its array-like characteristics.

You have correctly assigned a length property to the object, which is essential for mimicking an array.

By using the push method from Array.prototype, you were able to add an element to the array and increase the length property from 0 to 1. As a result, the length is now set to 1.

In the next step, you used literal notation to assign a property to the object, similar to something like customArray['someProperty'] = 1.

Since no method from Array.Prototype was utilized with this literal assignment, the customArray object did not recognize that it should behave like an array. Consequently, its length property remained unchanged, causing it to act solely as an object.

It's important to remember that the length property is unique to arrays and is properly managed by methods within the Array class.

Note: Creating array-like objects is not recommended, and it is your responsibility to manually handle indexing and other array-related operations for such objects.

Answer 4