如果你用var声明变量，你可以用var重新声明变量很多次，但是如果你用let或const重新声明，它会出错。用let和const，你只能声明一个变量。
您可能想要做的是重新分配变量，您可以使用var或let执行任意次数的重新分配，只是不要再次添加var或let。

重新声明

// valid, but not advised
var cube = (num) => num * num * num;
var cube = (num) => num * num * num;

// invalid
var cube = (num) => num * num * num;
let cube = (num) => num * num * num;

// invalid
var cube = (num) => num * num * num;
const cube = (num) => num * num * num;

重新分配

// valid
var cube = (num) => num * num * num;
cube = (num) => num * num * num;

// valid
let cube = (num) => num * num * num;
cube = (num) => num * num * num;

// invalid - can't change a const
const cube = (num) => num * num * num;
cube = (num) => num * num * num;