How to get Unique count based on property value in array of objects

I have an array as shown below:

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' }
];

Now I want to obtain a count of unique userId values. If a value is null, it should be counted separately for each occurrence. If a non-null userId value appears multiple times, it should only be counted once.

I tried the below function, But it does not consider null values

const uniqueItems = (list, key) =>
  list.reduce(
    (resultSet, item) =>
      resultSet.add(typeof key === "string" ? item[key] : key(item)),
    new Set()
  ).size;
uniqueItems(array, "userId");

You can use .reduce() to combine all elements into a single output.

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

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) => {
  // Skip if we already have the number.
  if(acc.cache.includes(obj.userId)) return acc;
  
  // Add new elements to cache.
  if(obj.userId !== null) acc.cache.push(obj.userId);
  
  // Increase count.
  acc.count++;
  
  return acc;
}, {cache: [], count: 0});

console.log(count);

<!-- end snippet -->

It may not be the most performant solution, but if the original array isn't huge, the easiest approach may be to separately count nulls and the unique non-null elements:

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;

You could map the null value with the index to have a unique identifier.

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

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: '19', userId: null },
  { _id: '10', userId: '2' }
];

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

console.log(uniqueItems(array, "userId"));

You can use Array.prototype.reduce() combined with Set

Code:

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

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)

<!-- end snippet -->

Just use .map and .reduce:

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