If you wanted to do O(n) functionally, you could do

    [1, 1, 2, 3, 4].filter(
      (obj => x => {
        if (obj[x]) {
          return false;
        }
        obj[x] = true;
        return true;
      })({}),
    );

What's happening is that you're passing an object (hashmap / hashset) into a function that returns a filtering function, and that object is used inside the closure to track the dupes. It's still a pure function because even though you're mutating the passed in object, the filtering function is still deterministic and referentially transparent.

The good old question about sacrificing performance for readability. Anyway these days filter is not so bad.

[1] https://jsperf.com/array-uniq-filter