Array.prototype.join
`${}`
+=
= '' + ''

var a = "aaaa"
var b = "bbbbb"

// 方法一： “+”
var c = a + b
console.log("c:", c)

// 方法二： “join("")”
var d = []
d.push(a,b)
console.log("d:", d.join(""))

// 方法三：模版字符串 `${}`
var e = `${a}${b}`
console.log("e:", e)

const a = 'aaaaa';
const b = 'bbbbb';

- a + b

- `${a}${b}`

- a.concat(b)

- 循环遍历组装