Comment by seliopou

The original line was something like this:

  acc = "";
  for (var i = 0; i < strings.length; i++) {
      acc += strings[i]
  }

The new version is something like this:

  strings.join("")

The second version can pre-allocate a string and copy characters into that string under the hood. The first version has to allocate a new string and recopy the characters on every iteration.