Get duplicate values of an Array

array.duplicate.js

// Most of this code is from an answer to a question in 
// Rebecca Murphey's JS Assessment project
// https://github.com/rmurphey/js-assessment (thanks, Rebecca!)

Array.prototype.duplicate = function() {

    var seen = {}, // Object to keep track of duplicate occurrences
        ret = [], // Our return array, with the duplicated values
        i, l; // iterator vars

    for (i = 0, l = this.length; i < l; i+=1) {

        // for each value, we create a has that gets incremented
        // if the value is duplicated.
        seen[this[i]] = seen[this[i]] ? seen[this[i]] + 1 : 1;

    }

    for (item in seen) {

        // Iterating over the properties of the seen object,
        // let's check if the the property is a member of the object
        // and if the associated value is greater than one (a duplicate)
        if (seen.hasOwnProperty(item) && seen[item] > 1) {

            // I like to convert numbers back to actual numbers
            // otherwise, the duplicate array will return numbers cast as strings
            item = parseFloat(item) ? parseFloat(item, 10) : item;

            // if it is, push it to our return array
            ret.push(item);

        }

    }

    return ret;

}