Trie implementation in JavaScript

trie.js

// Tries are implemented as objects containing values and maps of letters to a value or subtrie.

// Construct trie from given list of strings (optional, defaults to 0)
function Trie(strings) {
  this.value = null;
  this.nodes = {};
  
  if (strings !== undefined)
    for (var str in strings)
      this.addString(str);
};

// Add str to trie
Trie.prototype.addString = function(str, value) {
  var root = this;
  value = value || true; // default value is true
  
  for (var i in str) {
    if (!root.nodes.hasOwnProperty(str[i]))
      root.nodes[str[i]] = Trie(); // create new node corresponding to str[i]
    
    root = root.nodes[str[i]]; // traverse down
  }
  
  // set value of node
  root.value = value; 
};

// Get value of str, or null if str not in trie
Trie.prototype.valueOf = function(str) {
  var root = this;
  
  // traverse down trie
  for (var i in str) {
    if (!root.nodes.hasOwnProperty(str[i])) // can't traverse further
      return false;
    root = root.nodes[str[i]]; // traverse down
  }
  
  return root.value;
};

// Return matcher that, when applied to a string, replaces instances of a word s in trie with fun(s)
Trie.prototype.matcher = function(fun) {
  fun = fun || (str => this.valueOf(str))
  return str => {
    var len = str.length;
    var res = "";
    var found = false;
    
    for (var i = 0, j = 0; i < len; i++) {
      // traverse trie until a match or a dead end
      j = i;
      found = false;
      for (var root = this; j < len && root.nodes.hasOwnProperty(str[j]) && !found; j++, root = root.nodes[str[j]]) {
        if (root.value) { // found match
          res += fun(str.substring(i, j + 1));
          i = j; // move after match
          found = true;
        }
      }
      if (!found)
        res += str.charAt(i);
    }
    
    return res;
  };
};