插入数组

有两个数组
arr1 = [1, 2, 3, 4, 5, 6, 7, 8, 9]
arr2 = ['a', 'b', 'c']
在 arr1 中每隔4位插入 arr2 中的一位，不够4位的拼接在最后
插入后的结果为 1 2 3 4 a 5 6 7 8 b 9 c

const arr1 = [1, 2, 3, 4, 5, 6, 7, 8, 9];
const arr2 = ['a', 'b', 'c'];

const insertArr = function (arr1, arr2) {
  const insert = (arr1.length + arr2.length) / 4 | 0;
  let index = 4;
  for (let i=0; i<insert; i++) {
    arr1.splice(index, 0, arr2.shift());
    index += 5;
  }
  return [...arr1, ...arr2].join(' ');
}

insertArr(arr1, arr2); // 1 2 3 4 a 5 6 7 8 b 9 c

const arr1 = [1, 2, 3, 4, 5, 6, 7, 8, 9];
const arr2 = ['a', 'b', 'c'];

const insertArr = function (arr1, arr2) {
  const len = arr1.length + arr2.length;
  for (let i=0; i<len; i++) {
    if (i % 10 === 4 || i % 10 === 9) {
      arr1.splice(i, 0, arr2.shift());
    }
  }
  return [...arr1, ...arr2].join(' ');
}

insertArr(arr1, arr2); // 1 2 3 4 a 5 6 7 8 b 9 c