convert.js

"use strict"
var dict = {};
var appendToDict = (char) => {
    if (dict[char]) dict[char]++
    else dict[char] = 1;
}
var convert = (string) => {
    string.toLowerCase().split('').map(appendToDict);
    for (key of Object.keys(dict)) {
        string = string.replace(new RegExp(key, 'ig'), dict[key] % 2 == 0 ? '\$' : '#');
    }
    return string;
}