Creating array-like objects in JavaScript

Question

In JavaScript, there are objects that pretend to be arrays (or are \"array-like\"). Such objects are arguments, NodeLists (returned from get

Answer 1

The same question got into my mind as while we can use array like arguments parameter:

function arrayLike() {
  console.log(typeof arguments)
  console.log(arguments)
  console.log(Array.from(arguments))
}
arrayLike(1,2,3)

So, let's try creating our own array-like object:

let arrayLikeObject = {
  0: 1,
  1: 2
 }
 
 console.log(Array.from(arrayLikeObject))

Obviously, there's no length property defined so our arrayLikeObject will only return an empty array. Now, let's try defining a length property:

let arrayLikeObject = {
  length: 2,
  0: 1,
  1: 2
 }
 
 console.log(Array.from(arrayLikeObject))

What if length is set different?

let arrayLikeObject = {
  length: 1,
  0: 1,
  1: 2
 }
 
 console.log(Array.from(arrayLikeObject))
 // it will only return the value from first `0: 1`

let arrayLikeObject = {
  length: 5,
  0: 1,
  1: 2
 }
 
 console.log(Array.from(arrayLikeObject))
 // other 3 values will be printed as undefined

---

But, I don't want to convert it...

You actually wanted to create an array, not array-like object. The array-like object must be converted like you said:

Array.prototype.slice.call(arrayLikeObject)
// Or,
[].slice.call(arrayLikeObject)

If you do try to use array methods on array-like object, then you'll get type error:

let arrayLikeObject = {
  length: 5,
  0: 1,
  1: 2
 }

 console.log(arrayLikeObject.sort())

Thus, to use the array methods on arrayLikeObject, we need to convert it into array as we did in preceding examples using Array.from.

Otherwise, you simply need to create an array:

let arr = [1,2] // I don't mean, you don't know

---

Other consideration:

You can't use it as constructor:

let arrayLikeObject = {
    length: 1,
    slice: function () {
      return 1
    }
}

console.log(new arrayLikeObject) // Type error

In the following snippet, the result will be [undefined] as the length property is set to 1 but there's no 0 indexed property:

let arrayLikeObject = {
  length: 1,
  slice: function () {
    return 1
  }
}
console.log(Array.from(arrayLikeObject))

But if you set the length to 0, then the result will be an empty array [] because we're telling that we don't have any values in this array-like object.