How to create a list of unique items in JavaScript?

Question

In my CouchDB reduce function I need to reduce a list of items to the unique ones.

Note: In that case it\'s ok to have a list, it will be a small number of items

Answer 1

An alternative that's suitable for small lists would be to ape the Unix command line approach of sort | uniq:

    function unique(a) {
        return a.sort().filter(function(value, index, array) {
            return (index === 0) || (value !== array[index-1]);
        });
    }

This function sorts the argument, and then filters the result to omit any items that are equal to their predecessor.

The keys-based approach is fine, and will have better performance characteristics for large numbers of items (O(n) for inserting n items into a hashtable, compared to O(n log n) for sorting the array). However, this is unlikely to be noticeable on small lists. Moreover, with this version you could modify it to use a different sorting or equality function if necessary; with hash keys you're stuck with JavaScripts notion of key equality.

Answer 2

Commonly, the approach you used is a good idea. But I could propose a solution that will make the algorithm a lot faster.

function unique(arr) {
    var u = {}, a = [];
    for(var i = 0, l = arr.length; i < l; ++i){
        if(!u.hasOwnProperty(arr[i])) {
            a.push(arr[i]);
            u[arr[i]] = 1;
        }
    }
    return a;
}

As you can see we have only one loop here.

I've made an example that is testing both your and my solutions. Try to play with it.