配列の連結 concat スプレッド構文
const arr01 = ["one", "two", "three"];
const arr02 = ["four", "five", "six"];
console.log(arr01.concat(arr02)); // ["one", "two", "three", "four", "five", "six"]concat は新しい配列が返される
その他の新しい配列として返り値で返されるメソッド：join slice map filter
console.log(arr01); // ["one", "two", "three"]
console.log(...arr01); // one two three
console.log([arr01, arr02]); // [Array(3), Array(3)]
console.log([...arr01, ...arr02]); // ["one", "two", "three", "four", "five", "six"]