To make my point clear, here is a question. What is wrong with the following function that counts the number of unique strings in a list of strings?

    function count_unique_strings(l)
    {
        var m = {};

        l.forEach(function(s)
        {
            m[s] = true;
        });

        var count = 0;
        for(var k in m) ++count;

        return count;
    }

To see why this function is incorrect, run it with the following input:

    console.log(count_unique_strings(["alpha", "bravo", "bravo", "charlie"]));
    console.log(count_unique_strings(["alpha", "bravo", "bravo", "__proto__"]));