assign property to object in array

Question

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

score 0 · Accepted Answer · answered Sep 28 '22 at 17:13

0

There may be a problem with prevCount not being defined. Other than that, you're doing it right. Try logging data after the map call. However, when modifying items in an array directly, use forEach instead of map.

data.forEach(item => {
  item.foo = 'bar'
})

Use map if you want a duplicate array with things changed. This returns a new array with the key 'foo' added to each object.

data.map(item => {
  return { ...item, foo: 'bar' }
})

answered Sep 28 '22 at 17:13

jjroley

157
10

The second method kind of worked, but i get a weird object structure back. I edited the question with a screenshot. – mattisvensson Sep 28 '22 at 18:35
Use the spread operator on `meter` as well since you want to include the keys found in meter, not the meter object itself. `return {...item, ...meter}` – jjroley Sep 28 '22 at 18:45
It came out that the weird structure came from mongoose, [see here](https://stackoverflow.com/questions/54951260/spread-syntax-returns-unexpected-object). I didn't even considered that this could be a problem. – mattisvensson Sep 29 '22 at 09:45

score 0 · Answer 2 · answered Sep 28 '22 at 17:16

0

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,
  }
];
data = data.map((item, index) => {return {...item,foo:"bar"}})
console.log(data);

answered Sep 28 '22 at 17:16

jahanzeb naqvi

48
7

you just need to use map function and assign back to same or different to save the object in persistence state. – jahanzeb naqvi Sep 28 '22 at 17:18
This kind of worked, but i get a weird object structure back. I edited the question with a screenshot. – mattisvensson Sep 28 '22 at 18:34
That should have work its a weird output not expecting such thing. will require values dataElec list sample object similiar and prevCount variable. or Are you using any JS library. – jahanzeb naqvi Sep 28 '22 at 18:50

assign property to object in array

2 Answers2