Why an empty array of N length in javascript does not have indices?

Question

So I have created an empty array in Javascript

const x = Array(5);

Or

const x = new Array(5);

And if i check the length, Its 5 in both the cases but when I loop over it using .map then it did not give me the indices i.e 0,1,2,3,4

x.map((el, i) => i);

This should return [0,1,2,3,4]

And yes I have achieved this behaviour using const x = Array.from(Array(5)) but just wanted to know the reason why above approach is not working. — Sandeep Sihari
– Sandeep Sihari, Commented Dec 30, 2021 at 4:38
see also ... No operation for uninitialized values (sparse arrays) — Peter Seliger
– Peter Seliger, Commented Dec 30, 2021 at 5:15

CertainPerformance · Accepted Answer · 2021-12-30 04:41:15Z

2

Because the specification requires that when an array is created like that with the Array constructor, where values is the argument list:

a. Let len be values[0].
b. Let array be ! ArrayCreate(0, proto).
c. If Type(len) is not Number, then
  i. Perform ! CreateDataPropertyOrThrow(array, "0", len).
  ii. Let intLen be 1𝔽.
d. Else,
  i. Let intLen be ! ToUint32(len).
  ii. If SameValueZero(intLen, len) is false, throw a RangeError exception.
e. Perform ! Set(array, "length", intLen, true).
f. Return array.

That's all it does, and nothing else. In short, it creates a new array object inheriting from Array.prototype, and then sets a length property on it. It doesn't create any numeric indicies on the array.

It's like

const createArray = (length) => {
  const newArr = Object.create(Array.prototype);
  newArr.length = length;
  return newArr;
};

const arr = createArray(5);
console.log(arr.length);
console.log(arr.hasOwnProperty('0'));

As a result, if you want to iterate over the array, you'll have to fill it first somehow - either with .fill, Array.from, or spreading it into a new array..

answered Dec 30, 2021 at 4:41

CertainPerformance

373k55 gold badges354 silver badges359 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Sandeep Sihari Over a year ago

Thanks for the detailed answer this is really helpful. Can you also explain where this is useful? I mean a practical use-case?

CertainPerformance Over a year ago

I suppose, in rare cases, one would like to create an array with a given large length without also filling all those indicies with values. But that'd be really strange. It's best practice to avoid sparse arrays in all circumstances. I think, if the language was re-designed today, this behavior would be re-examined - it causes far more confusion than the benefits it provides.

Collectives™ on Stack Overflow

Why an empty array of N length in javascript does not have indices?

1 Answer 1

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related