let a1 = [1,2,3];

let a2 = [4,5,6];

a1.concat(a2); //结果 [1,2,3,4,5,6]

console.log(a1);//[1,2,3]

console.log(a2);//[4,5,6]

a2 = a2.concat(a1);//[4,5,6,1,2,3];

console.log(a2);//[4,5,6,1,2,3];

console.log(a1);[1,2,3];