duplicates.js

function getDuplicates(str)
{
    var duplicates = "";
    while (str.length > 0) {

        // Adds a placeholder for the string at the current position before we run replace
        var cur_str = str;

        // Searches for all instances of char in string and replaces them with nothing
        var regex = new RegExp(str[0], "g");
        str = str.replace(regex, "");

        // If the string differs by more than one we know that duplicates were found within the string
        if (cur_str.length - str.length > 1) {
            duplicates += cur_str[0];
        }

    }
    // Returns a string of duplicates
    return duplicates;
}