var arr1 = ['a', 'b', 'c'];

var 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);

//[2, 0, "1", "2", "3"]

Array.prototype.splice.apply(arr1, arr2);

console.log(arr1);

//["a", "b", "1", "2", "3", "c"]