1.concat(不改变原数组，concat合并数组之后，返回值才是新数组，可以合并两个以上数组)
let a = [1,2,3]
let b = [4,5,6]
let c = a.concat(b)
console.log(c) //[1,2,3,4,5,6]
console.log(a) //[1,2,3]
2.push(添加到数组的末尾，改变原数组)
let a = [1,2,3]
let b = [4,5,6]
for(let i = 0;i<b.length;i++){
a.push(b[i])
}
console.log(a) //[1,2,3,4,5,6]
3.push.apply(合并数组是把最后一个数组的值依次push进前一个数组，使前一个数组发生改变，并且只能两个数组之间发生合并)
let a = [1,2,3]
let b = [4,5,6]
a.push.apply(a,b)
console.log(a) //[1,2,3,4,5,6]
console.log(b) //[4,5,6]
4.ES6扩展运算符...(返回新数组)
let a = [1,2,3]
let b = [4,5,6]
let c = [...a,...b]
console.log(c) //[1,2,3,4,5,6]
console.log(a) //[1,2,3]
console.log(b) //[4,5,6]
