Simple options merger.

merge.js

module.exports = defaults;

function defaults (target, source) {
  target = target || {};
  source = source || {};
  var keys = Object.keys(source) || [];

  if (!keys.length) return target;

  if (Array.isArray(source)) {
    target = source;
    return target;
  }

  keys.forEach(function (key) {
    if (target[key] && typeof source[key] === 'object' && source[key] !== null) {
      target[key] = defaults(target[key], source[key]);
      return;
    }

    target[key] = source[key];
  });

  return target;
}

Better, tested module: https://github.com/nrf110/deepmerge