Discussion on: How to create range in Javascript

[Namir]

I like this way:

const range = (start, end) => {
    const length = end - start;
    return Array.from({ length }, (_, i) => start + i);
}

As array from works with arrayLike structures and receives a second optional argument mapFn, it's a nice candidate to build range.

[John]

Yes, excellent solution.

This, slimmed down, version also works...

const range = (start, end, length = end - start) =>
  Array.from({ length }, (_, i) => start + i)

[kerafyrm02]

const range = (start, end) => Array.from({length: end}, (_, i) => start + 1);

[John]

Sorry kerafyrm02, but that does not produce a range.

If I run range with range(1,50) I get [2,2,2,2,2,2,2,2,...]

const range = (start, end) => Array.from({length: end}, (_, i) => start + 1); console.log(range(0,20))

[kerafyrm02]

let r = (s, e) => Array.from('x'.repeat(e - s), (_, i) => s + i);

Ok here ya go... even shorter. :D

[John]

Rock, paper, scissors... 68 chars, you've nailed it!

[kerafyrm02]

lol., made range to r.. so even shorter :D

[John]

Trust me to bump into a js golfer.

Next you'll be telling me that you're dropping the let keyword and just leaving r on the global object!!

[YCM Jason]

haha! nice solution John!

[John]

Well, thank you so much for saying!

[Matt]

Almost.

Try:

const range = (start, end, length = end - start + 1) =>
  Array.from({ length }, (_, i) => start + i)

> range(0,2)
[ 0, 1, 2 ]

[Adam Catalfano]

as satisfying as this is, you're exposing a third argument that anyone can override. In a modern tech stack, this code is hesitantly admissible only if you can guarantee that absolutely no one can set length to be anything but end - start, otherwise you need to invest in a much more complex testing framework, likely outweighing the expense of adding a second line to this function.

[rionse09]

Hi All, I'm trying to grasp this function:

const range = (start, end) => {
    const length = end - start;
    return Array.from({ length }, (_, i) => start + i);
}

But I still confused on why does passing { length } as first argument in Array.from is work to create new array with specific length.

Because what I read in the documentation about Array.from is only works for iterable or array like object.

Any explanation or help would be appreciate a lot. Thank you.

[YCM Jason]

2 years too late to explain this. Hope it's not too late!

From the doc:

array-like objects (objects with a length property and indexed elements).

obj = { length: 10 } is:

• ✅ an object with a length property
• ✅ an object with indexed elements, where obj[i] is undefined for any integer i.

This is why { length } count as an "array-like" object. You could write in the long form:

{
  length: 3,
  0: undefined,
  1: undefined,
  2: undefined,
}

But it's not worth listing those entries out coz they are by default undefined anyways.