let and const

let

• let 可以用來宣告一個變數，並且將它限制在一定的區塊內，而非 global

if (true) {
  let x = 1;
  console.log(x); // 1
}

console.log(x); // undefined

• let 如果在程式的最上方或是 function 的最上方宣告變數，let 的行為和 var 的行為是一樣的

var x = 'global';
let y = 'global';

console.log(x); // global
console.log(y); // global

• 使用 let 重覆宣告同一變數將會得到 type error;

if (x) {
  let foo;
  let foo; // TypeError thrown.
}

• let vs var

var a = 5;
var b = 10;

if (a === 5) {
  let a = 4; // The scope is inside the if-block
  var b = 1; // The scope is inside the function

  console.log(a);  // 4
  console.log(b);  // 1
} 

console.log(a); // 5
console.log(b); // 1

const

• const 宣告的變數值不能改變，亦不能重覆宣告同一變數

const a; // SyntaxError: missing = in const declaration
const a = 5;
const a = 4; // trying to redeclare a constant throws an error 
var a = 3; // a is reserved for constant above, so this will also fail