I know there are a lot of questions like this and this is probably a duplicate, but be sure, i tried every single solution provided to these questions and none of them worked for me.

Here's the thing:

I have an array of objects:

let data = [
  {
    id: 1,
    year: 2022,
    month: 11,
    currentCount: 2460.9,
  },
  {
    id: 2,
    year: 2022,
    month: 9,
    currentCount: 2481.4,
  },
  {
    id: 3,
    year: 2022,
    month: 2,
    currentCount: 2521.1,
  }
]

and i want to loop through the array and add a new property to each object. I tried many different ways (forEach, for-loop, etc) but my latest try was this one:

data.map((item, index) => {
    if (prevCount === undefined) {
        //do some things here
    } else {

        //do some things here

        //the new property i want to add
        let newProperty = "foo"
        
        //some methods i tried
        item.newProperty = "foo"
        item[newProperty] = "foo"

        let obj = {newProperty: "foo"}
        let newObj = Object.assign(item, obj)

        //do some things here
    }
})

but in the end nothing happens. No error and no new property. I'm getting a bit crazy over this one, because it seems so simple...

Any help is appreciated

EDIT

After changing the map to this:

let newData = dataElec.map((item, index) => {
    if (prevCount === undefined) {
        prevCount = item.currentCount
        return item
    } else {
        let meter = {foo: 'bar'}
        return {...item, meter}
    }
})

it is kind of working, but i get a weird object structure:

the object and properties (first red box) are not added by my code. i don't know where they come from. The content/properties of the object "_doc" (second red box) should be on the same level as the "meter" object.

I could work with a structure like this, but i would like a clean array. And I'm wondering how the other objects are getting into my array anyways...

so again, any help is appreciated. Thanks