joinArray.js

export const joinArray = (list = [], separator) =>
  list.reduce((joinedList, item, index, {length}) => {
    joinedList.push(item);
    if (index < (length - 1)) {
      joinedList.push(separator);
    }
    return joinedList;
  }, []);

let list = [1, 2, 3, 4];
let joinedList = joinArray(list, 0);
console.log(joinedList);
// [1, 0, 2, 0, 3, 0, 4]