Remove Duplicates from an Array Using filter()

When we work large datasets, removing duplicates is a critical technique to work with unique values.

const items = ["apple", "banana", "apple", "orange", "banana", "kiwi"];

const uniqueItems = items.filter((item, index) => {
  return items.indexOf(item) === index;
});

console.log(uniqueItems);

How does this remove duplicates?

The `filter()` checks whether the current item’s index matches the **first occurrence** of that item in the array. If it doesn’t, it’s a duplicate and gets filtered out.

Is this case-sensitive?

Yes — `”Apple”` and `”apple”` are treated as different strings. If you want to make it case-insensitive, you can normalize the items:

const uniqueItems = items.filter((item, index, arr) =>
  arr.findIndex(i => i.toLowerCase() === item.toLowerCase()) === index
);

Can I use this with numbers too?

Yes! This approach works for arrays of numbers as well:

const numbers = [1, 2, 2, 3, 1, 4];
const uniqueNumbers = numbers.filter((n, i) => numbers.indexOf(n) === i);

Is this the most efficient way?

For small to medium arrays, yes. For larger datasets, using a `Set` is faster and cleaner:

const uniqueItems = [...new Set(items)];