https://developer.mozilla.org/ko/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce https://medium.com/@hongkevin/js-3-%EC%9E%90%EB%B0%94%EC%8A%A4%ED%81%AC%EB%A6%BD%ED%8A%B8-%EB%B0%B0%EC%97%B4-%EB%A9%94%EC%84%9C%EB%93%9C-reduce-100-%ED%99%9C%EC%9A%A9%EB%B2%95-feat-egghead-io-97c679857ece
