Create unique values from duplicates in Javascript array of objects

Question

I have an array of duplicated objects in Javascript. I want to create an array of unique objects by adding the index of occurrence of the individual value.

This is my initial data:

const array= [
  {name:"A"},
  {name:"A"},
  {name:"A"},
  {name:"B"},
  {name:"B"},
  {name:"C"},
  {name:"C"},
];

This is expected end result:

const array= [
  {name:"A-0"},
  {name:"A-1"},
  {name:"A-2"},
  {name:"B-0"},
  {name:"B-1"},
  {name:"C-0"},
  {name:"C-1"},
];

I feel like this should be fairly simple, but got stuck on it for a while. Can you please advise how I'd go about this? Also if possible, I need it efficient as the array can hold up to 1000 items.

EDIT: This is my solution, but I don't feel like it's very efficient.

const array = [
  { name: "A" },
  { name: "A" },
  { name: "C" },
  { name: "B" },
  { name: "A" },
  { name: "C" },
  { name: "B" },
];

const sortedArray = _.sortBy(array, 'name');

let previousItem = {
   name: '',
   counter: 0
};
const indexedArray = sortedArray.map((item) => {
  if (item.name === previousItem.name) {
    previousItem.counter += 1;
    const name = `${item.name}-${previousItem.counter}`;
    return { name };
  } else {
    previousItem = { name: item.name, counter: 0};
    return item;
  }
});

If they're sorted like that, it should just be a simple matter of using a counter variable, keeping track of the current letter, and then resetting the counter when you get a new letter. — user12407908
– user12407908, Commented Sep 26, 2020 at 23:59
posted my solution as an edit. thanks, the counter idea helped me to solve it — mhlavacka
– mhlavacka, Commented Sep 27, 2020 at 0:17

kind user · Accepted Answer · 2020-09-27 00:25:18Z

1

Currently you are sorting it first then looping over it, which may be not the most efficient solution.

I would suggest you to map over it with a helping object.

const a = [{name:"A"},{name:"A"},{name:"A"},{name:"B"},{name:"B"},{name:"C"},{name:"C"},], o = {};

const r = a.map(({ name }) => {
   typeof o[name] === 'number' ? o[name]++ : o[name] = 0;
   return { name: `${name}-${o[name]}` };
});
   
console.log(r);

edited Sep 27, 2020 at 0:25

answered Sep 27, 2020 at 0:18

kind user

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2 Comments

mhlavacka Over a year ago

I accepted your answer as the best solution as jsbench shows it as the fastest. I'm gonna break it down to understand how you used the helping object. thanks!

kind user Over a year ago

@mhlavacka The helping object basically stores the info how many times particular name was looped already.

Wyck · Accepted Answer · 2020-09-27 00:47:42Z

Keep a counter, and if the current name changes, reset the counter.

This version mutates the objects. Not sure if you want a copy or not. You could potentially sort the array by object name first to ensure they are in order (if that's not already an existing precondition.)

const array = [
  { name: "A" },
  { name: "A" },
  { name: "A" },
  { name: "B" },
  { name: "B" },
  { name: "C" },
  { name: "C" },
];

let name, index;
for (let i in array) {
  index = array[i].name == name ? index + 1 : 0;
  name = array[i].name;
  array[i].name += `-${index}`;
}

console.log(array);

Another way, if you don't want to sort, and don't want to mutate any objects, is to use a map and keep track of the current index for each object.

const array = [
  // NOTE: I put the items in mixed up order.
  { name: "A" },
  { name: "C" },
  { name: "A" },
  { name: "B" },
  { name: "A" },
  { name: "C" },
  { name: "B" },
];

let index = {};
let next = name => index[name] = index[name] + 1 || 0;
let result = array.map(obj => ({ ...obj, name: obj.name + '-' + next(obj.name) }));
console.log(result);

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