方法一:

const arr1 = ['a', 'b', 'c'];
const arr2 = ['1', '2', '3'];
// 把arr2 变成一个适合splice的数组（包含splice前2个参数的数组）
arr2.unshift(2, 0);
console.log(arr2);
Array.prototype.splice.apply(arr1, arr2);
console.log(arr1); 

方法二:

const arr1 = ['a', 'b', 'c'];
const arr2 = ['1', '2', '3'];
arr1.splice(2, 0, ...arr2)
console.log(arr1)