Increment "position" property by 1 for each matching property?

This is a bit of a tough problem to explain, so I’ll do my best.

I have two arrays, Categories and Items, and I’m attempting to build a third array called CategoryItems based off of those first two arrays. The function I currently have to build it looks like this:

const newCategoryItems = data.map(( item, i ) => {
  const category = categoriesData.find(c => c.name === item.category_name);

  return {
    item_id: bulkInsertData[i].id,
    category_id: category?.id,
    position: ???
  };
});
console.log({ newCategoryItems });

Each Category can have multiple Items. What I would like to do is assign that specific Item’s position within that Category. So for example, if Category 1 contains five Items, then their positions would increment from 1-5, and once Category 2 is detected then the next Items would start their position at 1, and so on.

Is this possible within my current function?

Edit
Here is some sample data:

raw data

const data = [
  {
    name: 'Item 1',
    price: 0,
    quantity: 1,
    url: null,
    user_id: 1,
    category_name: 'Category 1',
    qty: null
  },
  ... other raw data
]

I am then inserting those into my database and receiving these back with their assigned IDs.
categories:

const categoriesData = [
  {
    id: 148,
    name: 'Category 1',
    list_id: 2,
    updated_at: '2022-07-28T18:00:23.09351+00:00',
    created_at: '2022-07-28T18:00:23.09351+00:00',
  },
  {
    id: 149,
    name: 'Category 2',
    list_id: 2,
    updated_at: '2022-07-28T18:00:23.09351+00:00',
    created_at: '2022-07-28T18:00:23.09351+00:00',
  },
  ... other categories
]

items:

const bulkInsertData = [
  {
    id: 677,
    name: 'Item 1',
    price: 0,
    quantity: 1,
    url: null,
    user_id: 1
    created_at: '2022-07-28T18:00:22.810136+00:00',
    updated_at: '2022-07-28T18:00:22.810136+00:00',
  },
  {
    id: 678,
    name: 'Item 2',
    price: 0,
    quantity: 1,
    url: null,
    user_id: 1
    created_at: '2022-07-28T18:00:22.810136+00:00',
    updated_at: '2022-07-28T18:00:22.810136+00:00',
  },
  {
    id: 679,
    name: 'Item 3',
    price: 0,
    quantity: 1,
    url: null,
    user_id: 1
    created_at: '2022-07-28T18:00:22.810136+00:00',
    updated_at: '2022-07-28T18:00:22.810136+00:00',
  },
  ...other items
]

>Solution :

This would work. I’ve edited the test data by adding category directly to the first array to make this easier/more clear to reproduce. You could probably make an even more terse version using array.reduce().

const items = [
  {
    id: 677,
    name: 'Item 1',
    price: 0,
    quantity: 1,
    url: null,
    user_id: 1,
    created_at: '2022-07-28T18:00:22.810136+00:00',
    updated_at: '2022-07-28T18:00:22.810136+00:00',
    category: 'test1'
  },
  {
    id: 678,
    name: 'Item 2',
    price: 0,
    quantity: 1,
    url: null,
    user_id: 1,
    created_at: '2022-07-28T18:00:22.810136+00:00',
    updated_at: '2022-07-28T18:00:22.810136+00:00',
    category: 'test2'
  },
  {
    id: 679,
    name: 'Item 3',
    price: 0,
    quantity: 1,
    url: null,
    user_id: 1,
    created_at: '2022-07-28T18:00:22.810136+00:00',
    updated_at: '2022-07-28T18:00:22.810136+00:00',
    category: 'test1'
  }
];

const categoryCounter = {};
const newCategoryItems = items.map(( item ) => {

  if ( categoryCounter.hasOwnProperty( item.category ) ) {
    categoryCounter[item.category]++;
  } else {
    categoryCounter[item.category] = 1;
  }
    
    
  return {
    item_id: item.id,
    category_id: item.category,
    position: categoryCounter[item.category]
  };
});

console.log( newCategoryItems );

https://stackoverflow.com/questions/73170086/increment-position-property-by-1-for-each-matching-property#