Get index of Object inside Array and use it in a function

Question

In Javascript, is it possible to get the Index of the object (in an array of objects) and use it inside of the same object in a function?

For example:

const foo = [
  {
    id: 1,
    comments: getComments(this.index) // index of the object goes here
  },
  {
    id: 34,
    comments: getComments(this.index) // index of the object goes here
  },
  {
    id: 21,
    comments: getComments(this.index) // index of the object goes here
  }
]

const getComments = function(index) {
  return foo[index].id // return the ID of the actual object
}

Or is there a workaround for what I'm trying to achieve here?

At the time you're calling the function, the array doesn't even exist yet. — Barmar
– Barmar, Commented Jan 28, 2021 at 19:53
Also getComments won't exist when the array is defined. (I am confused) — evolutionxbox
– evolutionxbox, Commented Jan 28, 2021 at 19:55
If this worked, you would just be setting comments to the same thing as the id property. What's the point of that? — Barmar
– Barmar, Commented Jan 28, 2021 at 19:56

Barmar · Accepted Answer · 2021-01-28 20:00:06Z

1

Do it in two steps. First create the array of objects, then fill in the comments property by calling the function on each element.

const foo = [
  {
    id: 1,
  },
  {
    id: 34,
  },
  {
    id: 21,
  }
]

foo.forEach(el => el.comments = getComments(el));

function getComments(obj) {
  return obj.id;
}

console.log(foo);

answered Jan 28, 2021 at 20:00

Barmar

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1 Comment

Radical_Activity Over a year ago

Really nice, it makes sense to run a function after the array has been created. How silly of me to not thinking of that. Thanks for pointing it out.

Jose Marin · Accepted Answer · 2021-01-28 20:28:59Z

1

I assume this is for learning purposes as probably the use you are thinking could be implemented in a better way. The closest that you can do is this:

let foo = [
  {
    id: 1,
  },
  {
    id: 34,
  },
  {
    id: 21,
  }
]

foo = foo.map(
  (elem, index) => ({...elem, comments: getComments(index)})
)

function getComments(index) {
  return foo[index].id // return the ID of the actual object
}

console.log(foo[2].comments)

answered Jan 28, 2021 at 20:28

Jose Marin

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