Capitalize first letter of every word in a string in Javascript

capitalize.js

capitalizeString = str => {
    str = str.toString();
    str = str.split(" ");

    for (var i = 0, x = str.length; i < x; i++) {
      str[i] = str[i][0].toUpperCase() + str[i].substr(1);
    }

    return str.join(" ");
  };