编辑代码

var arr1 = [1, 2, 3];
var arr2 = [4, 5, 6];

// concat方法
console.log(arr1.concat(arr2));

// apply方法
function mergeArr (a1, a2) {
    a1 = [...a1];
    a2 = [...a2];
    a1.push.apply(a1, a2)
    return a1;
    
}
console.log(mergeArr(arr1, arr2));

// 扩展运算符
console.log([...arr1, ...arr2]);

// for循环
function mergeArr2 (a1, a2) {
    a1 = [...a1];
    for (let i = 0; i < a2.length; i++) {
        a1.push(a2[i]);
    }
    return a1;
}
console.log(mergeArr2(arr1, arr2));