constlet 宣告
與 var 不同，const / let 與詞法範圍而非功能範圍繫結。
{
var x = 1 // will escape the scope
let y = 2 // bound to lexical scope
const z = 3 // bound to lexical scope, constant
}
console.log(x) // 1
console.log(y) // ReferenceError: y is not defined
console.log(z) // ReferenceError: z is not defined