How to create a list of unique items in JavaScript?

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.

The best method seem to be using ES6 and Set. Single line and faster* than above according to fiddle

    
const myList = [1,4,5,1,2,4,5,6,7];
const unique = [...new Set(myList)];
    
console.log(unique);

*tested in safari

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.