Implementation of Trie data structure in javascript

trie.js

function Node() {
    this.keys = new Map();
    this.end = null;
    this.setEnd = function() {
        this.end = true;
    }
    this.isEnd = function() {
        return this.end;
    }
}

function Trie() {
    this.root = new Node();
    
    this.add = function(word, node = this.root) {
        if (word) {
            if (!node.keys.has(word[0])) {
                node.keys.set(word[0], new Node())
                return this.add(word.substr(1), node.keys.get(word[0]));
            } else {
                return this.add(word.substr(1), node.keys.get(word[0]))
            }
        }

        node.setEnd();
        return;
    }

    this.isWord = function(word, node = this.root) {
        while (word.length > 1) {
            if (!node) return false;

            if (node.keys.has(word[0])) {
                node = node.keys.get(word[0]);
                word = word.substr(1);
            } else {
                return false;
            }
        }

        return !!(node.keys.has(word) && node.keys.get(word).end);
    }

    this.print = function() {
        const words = [];

        let search = function(node, str) {
            for (let key of node.keys.keys()) {
                if (!key) return;

                let l = str.concat(key);

                if (node.keys.get(key).end) {
                    words.push(l);
                }

                search(node.keys.get(key), l);
            }
        }

        search(this.root, "");

        return words;
    }
}

const s = new Trie();

s.add('ball'); 
s.add('bat'); 
s.add('doll'); 
s.add('dork'); 
s.add('do'); 
s.add('dorm')
s.add('send')
s.add('sense')
console.log(s.isWord('dollaaa'))
console.log(s.isWord('dor'))
console.log(s.isWord('dorf'))
console.log(s.print())