🈯lexical declaration

🚧 施工中

JS ⟩ grammar ⟩ declaration ⟩ lexical declaration

lexical declarations (let/ const/ class)❓

🧨 雷區

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

🔴 主題

• temporal dead zone

• SyntaxError：lexical declaration cannot appear in a single-statement context❗️❗