题解 | #添加元素#

function insert(arr, item, index) {
            const newArr = []
            for (let i = 0; i < arr.length; i++) {
                if(i===index) {
                    newArr.push(item)
                    newArr.push(arr[i])
                }else {
                    newArr.push(arr[i])
                }
            }

            return newArr;
        }