es6-let_const.md

ES6 - (Let + Const)

NodeJS => ES6支援程度

• shipping (開發完成並默認支持)
• staged (開發完成，但必須使用--harmony參數)
• in progress (開發中)

let

let宣告的變數， 作用域僅限於該區塊 {}以內， 不能重複宣告

let x = 3; //在最外層宣告等同於var x = 3
var z = 10; 
{
    let y = 5;
    var z = 3;
    console.log(`x = ${x}, y = ${y}, z = ${z}`); // x = 3, y = 5, z = 3
    // let y = 3; // 'y' has already been declared
}
console.log(`x = ${x}, y = ${y}`); // y is not defined

const

const宣告的值無法改變

const x = 5;

x = 3; //TypeError: Assignment to constant variable
var x = 4; //SyntaxError: Identifier 'x' has already been declared