Obtaining the count of distinct values for a specific property in an array of objects

I have a unique array structure as follows:

const uniqueArray = [
  { _id: '1', userId: '5' },
  { _id: '2', userId: null },
  { _id: '3', userId: null },
  { _id: '4', userId: '1' },
  { _id: '5', userId: '2' },
  { _id: '6', userId: '4' },
  { _id: '7', userId: '4' },
  { _id: '8', userId: null },
  { _id: '9', userId: null },
  { _id: '10', userId: '2' }
];

I would like to find out the total number of unique userId values. If a value is null, each occurrence should be counted separately. If a non-null userId value is repeated, it should only be counted once.

I attempted the function below, but it does not consider null values

const getUniqueCount = (arr, key) =>
  arr.reduce(
    (set, item) =>
      set.add(typeof key === "string" ? item[key] : key(item)),
    new Set()
  ).size;
getUniqueCount(uniqueArray, "userId");
javascriptarraysobject

Answer №1

To consolidate all the elements into a single output, you can utilize the .reduce() method.

const array = [
  { _id: '1', userId: '5' },
  { _id: '2', userId: null },
  { _id: '3', userId: null },
  { _id: '4', userId: '1' },
  { _id: '5', userId: '2' },
  { _id: '6', userId: '4' },
  { _id: '7', userId: '4' },
  { _id: '8', userId: null },
  { _id: '9', userId: null },
  { _id: '10', userId: '2' }
];

const {count} = array.reduce((acc, obj) => {
  if(acc.cache.includes(obj.userId)) return acc;
  
  if(obj.userId !== null) acc.cache.push(obj.userId);
  
  acc.count++;
  
  return acc;
}, {cache: [], count: 0});

console.log(count);

Answer №2

While it may not be the most efficient solution, for smaller original arrays, a simple approach would be to count the number of null values and unique non-null elements separately:

const numNulls = array.filter(x => x.userId === null).length;
const numUniqueNotNull = new Set(array.map(x => x.userId).filter(u => u !== null)).size
const totalSize = numNulls + numUniqueNotNull;

Answer №3

If you want to assign a unique identifier to the null values in the array, you can map them with their corresponding index.

new Set(array.map((item,index) => (item.userId === null ? `${item.userId}-${index}` : item.userId))).size

Answer №4

const items = [
  { _id: '1', userId: '5' },
  { _id: '2', userId: null },
  { _id: '3', userId: null },
  { _id: '4', userId: '1' },
  { _id: '5', userId: '2' },
  { _id: '6', userId: '4' },
  { _id: '7', userId: '4' },
  { _id: '8', userId: null },
  { _id: '19', userId: null },
  { _id: '10', userId: '2' }
];

const findUniqueItems = (list, key) => {
  let occurrences = {}; 
  let count = 0;
  items.map((item, index) => {
    count += typeof item[key] == "string" ? ((occurrences[item[key]]) ? 0 : 1) : 1;
    occurrences[item[key]] = 1;
  });
  return count;
};

console.log(findUniqueItems(items, "userId"));

Answer №5

Utilize Array.prototype.reduce() in conjunction with Set for an effective solution

Here is the code snippet:

const array = [
  { _id: '1', userId: '5' },
  { _id: '2', userId: null },
  { _id: '3', userId: null },
  { _id: '4', userId: '1' },
  { _id: '5', userId: '2' },
  { _id: '6', userId: '4' },
  { _id: '7', userId: '4' },
  { _id: '8', userId: null },
  { _id: '9', userId: null },
  { _id: '10', userId: '2' },
]

const { users, nulls } = array.reduce(
  (a, { userId: id }) => (id ? a.users.add(id) : a.nulls++, a),
  { users: new Set(), nulls: 0 }
)

console.log('Total:', users.size + nulls)

Answer №6

To achieve this, simply utilize .map alongside .reduce:

array.map(item => item.userId)
  .reduce((prev, cur) => !prev.includes(cur)? [...prev, cur]: prev, [])
  .length

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