join - 배열 구분자
const join = (arr, separator = ',', end = separator) => arr.reduce( (acc, val, i) => i === arr.length - 2 ? acc + val + end : i === arr.length - 1 ? acc + val : acc + val + separator, '' ); join(['pen', 'pineapple', 'apple', 'pen'], ',', '&'); // "pen,pineapple,apple&pen" join(['pen', 'pineapple', 'apple', 'pen'], ','); // "pen,pineapple,apple,pen" join(['pen', 'pineapple', 'apple', 'pen']); // ..