a和b值交换
要求a = 20 , b = 10
方法一：定义一个变量c来传值
let a = 10
let b = 20
let c = a
a = b
b = c
console.log(a,b)
方法二：自身加和，重新赋值
let a = 10
let b = 20
a = a + b //30
b = a - b //10
a = a - b //20
console.log(a,b)
方法三：ES6语法
//此时必须用var来声明变量，因为var可以重复定义，let只能定义一次
var a = 10
var b = 20
var [b, a] = [a, b]
console.log(a,b)
