Get unique item from two different array in JavaScript

We are required to make a function that accepts an array of arrays, and returns a new array with all elements present in the original array of arrays but remove the duplicate items.

For example ? If the input is ?

const arr = [
   [12, 45, 65, 76, 76, 87, 98],
   [54, 65, 98, 23, 78, 9, 1, 3],
   [87, 98, 3, 2, 123, 877, 22, 5, 23, 67]
];

Then the output should be single array of unique elements like this ?

[
   12, 45, 54, 78, 9,
   1, 2, 123, 877, 22,
   5, 67
]

Using indexOf and lastIndexOf

This approach flattens the arrays and filters out elements that appear more than once by comparing their first and last occurrence positions.

const arr = [
   [12, 45, 65, 76, 76, 87, 98],
   [54, 65, 98, 23, 78, 9, 1, 3],
   [87, 98, 3, 2, 123, 877, 22, 5, 23, 67]
];

const getUnique = (arr) => {
   const newArray = [];
   arr.forEach((el) => newArray.push(...el));
   return newArray.filter((item, index) => {
      return newArray.indexOf(item) === newArray.lastIndexOf(item);
   });
};

console.log(getUnique(arr));

[
   12, 45, 54, 78, 9,
   1, 2, 123, 877, 22,
   5, 67
]

Using Set and Filter

This method uses a Set to track seen elements and filters out duplicates during the flattening process.

const arr = [
   [12, 45, 65, 76, 76, 87, 98],
   [54, 65, 98, 23, 78, 9, 1, 3],
   [87, 98, 3, 2, 123, 877, 22, 5, 23, 67]
];

const getUniqueWithSet = (arr) => {
   const flattened = arr.flat();
   const counts = {};
   
   // Count occurrences
   flattened.forEach(item => {
      counts[item] = (counts[item] || 0) + 1;
   });
   
   // Return items that appear only once
   return flattened.filter((item, index) => {
      return counts[item] === 1 && flattened.indexOf(item) === index;
   });
};

console.log(getUniqueWithSet(arr));

[
   12, 45, 54, 78, 9,
   1, 2, 123, 877, 22,
   5, 67
]

Comparison

Conclusion

Both methods effectively extract unique elements from nested arrays. The Set-based approach offers better performance for large datasets, while the indexOf method is more straightforward to understand.