cute ghost idea

ghost.js

//////////////////
// basic Trie implementation
//////////////////

function Trie() {
    this.head = { key: '', children: {} }
}

Trie.prototype.add = function (key) {

    var curNode = this.head
        , newNode = null
        , curChar = key.slice(0, 1);

    key = key.slice(1);

    while (typeof curNode.children[curChar] !== "undefined" && curChar.length > 0) {
        curNode = curNode.children[curChar];
        curNode.wins = key.length % 2 === 1 || curNode.wins === true;
        curChar = key.slice(0, 1);
        key = key.slice(1);
    }

    while (curChar.length > 0) {
        newNode = { key: curChar, wins: key.length % 2 === 1, children: {} };

        curNode.children[curChar] = newNode;

        curNode = newNode;

        curChar = key.slice(0, 1);
        key = key.slice(1);
    }

}
    ;

//////////////////
function ghost(dictionary = []) {
    const myTrie = new Trie();
    dictionary.forEach(word => myTrie.add(word));
    return Object.values(myTrie.head.children).filter(({ wins }) => wins).map(({ key }) => key)
}

function simpleGhost(dictionary = []) {
    const myTrie = {};
    dictionary.forEach(word => {
        const entry = myTrie[word[0]];
        myTrie[word[0]] = (entry === undefined || entry === true) && word.length % 2 === 0;
    }
    );
    return Object.entries(myTrie).filter(([k, wins]) => wins).map(([key]) => key)
}