const getPropValueByStringPath = (target, str) => {
  const path = str.split('.');
  const key = path.shift();

  if(path.length !== 0 && target[key] instanceof Object)
    return getPropValueByStringPath(target[key], path.join('.'));
  else if(path.length === 0)
    return target[key];
}

// example
const target = {
  a: {
    b: {
      c: {
        d: 8
      }
    }
  }
}

getPropValueByStringPath(target, 'a.b.c.d'); //returns 8