Trie implementation in ES6, good for string autocomplete

Trie.js

/* Class representing a Trie data structure */
export default class Trie {

  /**
   * Creates a Trie
   * @return {Object} Trie
   */
  constructor() {
    this.words = 0;
    this.prefixes = 0;
    this.children = [];
  }

  /**
   * Insert a string into the Trie
   * @param  {String} str String to add
   * @param  {Number} pos Optional position in Trie
   * @return {}
   */
  insert (str, pos = 0) {
    if (str.length === 0) {
      return;
    }
    if (pos === str.length) {
      this.words++;
      return;
    }
    this.prefixes++;
    const k = str[pos];
    if (this.children[k] === undefined) {
      this.children[k] = new Trie();
    }
    const child = this.children[k];
    child.insert(str, pos + 1);
  }

  /**
   * Return all words in Trie with a given prefix
   * @param  {String} str Prefix to search for
   * @return {Array} Array of strings that match for prefix
   */
  getAllWords (str = '') {
    /* Array of prefixs we are iterating through */
    let wordStack = [];

    /* if this current leaf child, end */
    if (this === undefined) {
      return [];
    }

    /* Check to see if this child node has words left */
    if (this.words > 0) {
      wordStack.push(str);
    }

    /* Iterate through all children and build up the prefixes to the wordStack */
    for (const k in this.children) {
      if ({}.hasOwnProperty.call(this.children, k)) {
        const child = this.children[k];
        wordStack = wordStack.concat(child.getAllWords(str + k));
      }
    }
    return wordStack;
  }

  /**
   * Remove a string from Trie
   * @param  {String} str String to remove
   * @param  {Number} pos Optional position to remove
   * @return {}
   */
  remove (str, pos = 0) {
    if (pos === str.length) {
      this.words--;
    }
    if (str.length === 0 || this === undefined) {
      return;
    }
    this.prefixes--;
    const k = str[pos];
    const child = this.children[k];
    if (child) {
      child.remove(str, pos + 1);
    }
  }

  /**
   * Returns the autoComplete object
   * @param  {String} str String to search for
   * @param  {Number} pos Optional position
   * @return {Object} Returns an object where prev is the current string that is
   * being searched and found is the array of matching words
   */
  autoComplete (str, pos = 0) {
    if (str.length === 0) {
      return {};
    }
    const k = str[pos];
    const child = this.children[k];

    if (child === undefined) {
      return {};
    }
    if (pos === str.length - 1) {
      return { prev: str, found: child.getAllWords(str) };
    }
    return child.autoComplete(str, pos + 1);
  }
}