Implement Trie (Prefix Tree) LeetCode solution in JavaScript

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var Trie = function () {
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  this.children = {};
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  this.word = false;
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};
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/**
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 * @param {string} word
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 * @return {void}
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 */
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Trie.prototype.insert = function (word) {
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  if (!word) {
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    this.word = true;
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    return null;
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  }
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  let head = word[0];
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  let tail = word.substring(1);
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  if (!this.children[head]) {
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    this.children[head] = new Trie();
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  }
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  this.children[head].insert(tail);
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};
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/**
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 * @param {string} word
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 * @return {boolean}
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 */
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Trie.prototype._search = function (word, method) {
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  if (!word) {
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    if (method === "search") {
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      return this.word;
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    } else {
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      return true;
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    }
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  }
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  let head = word[0];
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  let tail = word.substring(1);
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  if (!this.children[head]) {
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    return false;
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  }
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  return this.children[head][method](tail);
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};
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/**
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 * @param {string} word
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 * @return {boolean}
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 */
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Trie.prototype.search = function (word) {
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  return this._search(word, "search");
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};
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/**
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 * @param {string} prefix
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 * @return {boolean}
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 */
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Trie.prototype.startsWith = function (prefix) {
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  return this._search(prefix, "startsWith");
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};
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/**
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 * Your Trie object will be instantiated and called as such:
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 * var obj = new Trie()
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 * obj.insert(word)
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 * var param_2 = obj.search(word)
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 * var param_3 = obj.startsWith(prefix)
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 */