How would I replicate nth-child in JavaScript

as stated in the title I'm trying to do what css does but in JS using only an index of an item in a mapped array of items, each item get's an index number.

computed: {
        parsedItems() {
            return this.items?.map((obj, index) => {
                return {
                    ...obj,
                }
            })
        }
    }

For better understanding I'll try to explain why I'm doing this.

I'm building a grid of blocks that have some data (image, title, uri, some text, ...etc), and I want to get the functionality of :nth-child(7n + 1), and obviously all the way through to + 7.
But using JS, the framework is vue 2.

Parsed items have images in the obj and a custom component is using those images, that custom component is made to handle all kinds of stuff with images including aspect ratio of the image.
The component just takes the result from height divided by width of the image and uses that number to set the aspect ratio.

After extensive research I learned how nth-child works in css,

:nth-child(3n + 3)

Looks like this behind the scenes:

(3 x 0) + 3 = 3 = 3rd Element
(3 x 1) + 3 = 6 = 6th Element
(3 x 2) + 3 = 9 = 9th Element
etc.

What I tried to do is the following,
most obviously just multiplying index by 7 and adding to that number 1 through 7. like so:

computed: {
        parsedItems() {
            return this.items?.map((obj, index) => {
                return {
                    ...obj,
                    aspectRatio:
                        7 * index + 1 === 1
                            ? someAspectRatio
                            : 7 * index + 2 === 9
                                ? someAspectRatio
                                ...etc
                }
            })
        }
    }

Problem here is the map method doesn't stop at 7 and then starts it from 1 again, so this flunked.
Also, the math doesn't work out to what I need..
Second attempt was using the modulo/remainder operator "%".

computed: {
        parsedItems() {
            return this.items?.map((obj, index) => {
                return {
                    ...obj,
                    aspectRatio:
                        index % 7 === 0
                            ? someAspectRatio
                            : index % 3 === 0
                                ? someAspectRatio
                                : index % 2 === 0
                                    ? someAspectRatio
                                    : someAspectRatio
                }
            })
        }
    }

I found this to be closer but not close enough, as the module results start mixing up as the number of items increases.

The easiest way to make it work is to use simple for instead of map().

function nthChild(array, startIndex, eachIndex) {
   let newArray = []
   for(let i = startIndex; i < array.length; i+=eachIndex) {
       newArray.push(array[i])
   }
   return newArray
}

So from CSS reference :nth-child(eachIndex*n + startIndex)

Here is an example:

let ar = [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]
nthChild(ar, 3, 3)

output: (6) [3, 6, 9, 12, 15, 18]