gistfile1.js

function Counter(str) {
    var filter_instance = this; // keep a reference to this object's instance
    this.char_count     = 0;    // number of unique chars found

    /* return a table mapping each char to it's number of occurrences */
    function count_chars() {
        var array = str.split("");
        var table = {};
        array.map(function(elm, index, a) {
            /* this function is called in the context of `array`. Which means
             * that we can't access `char_count` here using `this.char_count`,
             * that would actually access `array.char_count`, which would
             * return undefined!
             * 
             * So, we use the reference we saved before instead. 
             */
            if (!table[elm]) { // new unique character
                filter_instance.char_count++;
                table[elm] = 1 }
            else {
                table[elm]++ }
        });
        return table;
    }

    this.count_chars = count_chars;
}

var my_counter = new Counter("some string of characters");
console.log(my_counter.count_chars());
console.log(my_counter.char_count);