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 of string type.

My current way is to set keys of a object, then return the keys of that object
since the place the code can’t use things like _.uniq for example.

I’d like to find a more elegant way to spell it than this.

function(keys, values, rereduce) {
  // values is a Array of Arrays
  values = Array.concat.apply(null, values);
  var uniq = {};
  values.forEach(function(item) { uniq[item] = true; });
  return Object.keys(uniq);
}

Answer 1

2021 answer:

const unique = (arr) => [...new Set(arr)];
unique([1, 2, 2, 3, 4, 4, 5, 1]); // [1, 2, 3, 4, 5]

Here you just create a set from the given array and then convert it back to the array.
I measured performance and it’s almost twice faster now than the approach proposed in the old answer I posted before. Also, it’s just a one-liner.

Updated fiddle

Old answer just for the record:

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.