const a = [1, 2, 'c', 4, 5]; a[2] = 3; // 3番目の要素に代入 console.log(a) // [1, 2, 3, 4, 5] console.log(a.length); // 5 a[81.3] = "eighty-one point three"; console.log(a[81.3]); // eighty-one point three a[-2] = "マイナス2"; console.log(a[-2]); // マイナス2 console.log(a); // [ 1, 2, 3, 4, 5, '81.3': 'eighty-one point three', '-2': 'マイナス2' ] console.log(a.length); // 5