function optionalAccess(obj, path, def) {
  const propNames = path.replace(/\]|\)/, "").split(/\.|\[|\(/);
  return propNames.reduce((acc, prop) => acc[prop] || def, obj);
}

const obj = {
  items: [
    { hello: "Hello" }
  ]
};

console.log(optionalAccess(obj, "items[0].hello", "def")); // => Hello
console.log(optionalAccess(obj, "items[0].he", "def")); // => def