Implementation of trie (data structure) in JS

trie.js

const input = 'feli';
const input2 = 'fela';

const head = {};

const add = (s, node, index) => {
  if (index === s.length) return;
  const char = s[index];
  if (!!node[char]) {
    node[char].size++;
  } else {
    node[char] = {
      size: 1
    };
  }
  add(s, node[char], ++index);
};

const find = (s, node, index) => {
  if (index === s.length) return node.size;
  const char = s[index];
  if (!node[char]) return 0;
  return find(s, node[char], ++index);
};

add(input, head, 0);
add(input2, head, 0);

console.log(JSON.stringify(head));
console.log(find('fel', head, 0));
console.log(find('fela', head, 0));
console.log(find('feli', head, 0));
console.log(find('fel1', head, 0));
console.log(find('felad', head, 0));

this is the implementation of a contact list, the contact list is made by two inputs ('feli' and 'fela') the contact list uses trie to find substring and return how many contacts satisfy my search.