1.改变原数组

（1）push+扩展运算法

利用push搭配扩展运算符的方法：

arr2经过扩展运算符，由[3,4,5]变成3,4，5
再用push将其添加到arr1的尾部

let arr1 = [0, 1, 2];
let arr2 = [3, 4, 5];
arr1.push(...arr2);
console.log(arr1)//[0, 1, 2, 3, 4, 5]

如果不适用扩展运算符，push会直接将arr2整体加入arr1的尾部

let arr1 = [0, 1, 2];
let arr2 = [3, 4, 5];
arr1.push(arr2);
console.log(arr1)//[0, 1, 2, [3, 4, 5]]

1.不改变原数组

（1）concat

let arr1 = [0, 1, 2];
let arr2 = [3, 4, 5];
let arr3=arr1.concat(arr2);
console.log(arr3)//[0, 1, 2, 3, 4, 5]
console.log(arr1)//[0, 1, 2]

（2）扩展运算符

let arr1 = [0, 1, 2];
let arr2 = [3, 4, 5];
let arr4=[...arr1,...arr2];
console.log(arr1)//[0, 1, 2,]
console.log(arr4)//[0, 1, 2, 3, 4, 5]