🈯const

• JS ⟩ variable ⟩ const

• JS ⟩ declaration ⟩ variable ⟩ const

⭐️ ES6 (2015)

(declaration) declares a local constant in block scope.

🧨 雷區

const

• must have an initial value❗ (👉 more)

• can't be reassigned❗ (👉 more)

• can't be redeclared❗

⛔ ReferenceError: cannot access '...' before initialization.

• caused by referencing a let/ const/ class in its temporal dead zone❗

✅ it's common to declare const using capital letters.

using typeof on lexical declaration (let/ const/ class) in its temporal dead zone will throw a ReferenceError.

// ⭐ temporal dead zone (start)
// ------------------------------
typeof aLet;       // ⛔ ReferenceError: Cannot access 'aLet' before initialization
typeof aConst;     // ⛔ ReferenceError: Cannot access 'aConst' before initialization
typeof aClass;     // ⛔ ReferenceError: Cannot access 'aClass' before initialization

typeof undeclared;        // ❗ 'undefined'

// ⭐ lexical declarations (let/const/class)
// --------------------------------------------
let aLet;                 // ⭐ initialized to `undefined`
const aConst = "hello";
class aClass {}

• replit：TDZ: let/const/class

⭐️ 重點

const

• is a reserved word❗
• must be initialized at declaration. 👉 const requires initialization

• can't be reassigned. 👉 const can't reassign

• can't be redeclared.

const

• can be used with for-in / for-of loop.

• can't be used with a classic for-loop.

👉 no const for classic "for"❗

🔴 主題

• const requires initialization❗
• const can't reassign❗
• no const for classic "for"❗
• temporal dead zone❗

👥 相關

• compare： let, var

📘 手冊

• Statements and declarations ⟩ declaring variables ⟩ const