🈯let

declares a block-scoped local variable.

• JS ⟩ variable ⟩ let

• JS ⟩ declaration ⟩ variable ⟩ let

⭐️ ES6 (2015)

(declaration) declares a local variable in block scope.

🧨 雷區

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

• typeof let/const/class in TDZ gets an error❗️

🔴 主題

• ❗global let shadows global object property❗
• ✅let redeclaration not allowed even in sloppy mode❗️
• ✅let can't shadow parameter even in sloppy mode❗️

⭐️ 重點

let is

• a contextual keyword,

• not reserved word❗

JS is lenient with var, strict with let.

• var redeclaration applied even in strict mode❗️, even in strict mode❗
• let redeclaration not allowed even in sloppy mode❗️, even in sloppy mode❗

雖說似乎有一個奇怪的 "for-init" scope，但裡面 let 變數的表現其實更像是 block-scoped。👉 scopes matter with closures ⟩ ❓

👥 相關

• ⚖️const / var
• ⛔temporal dead zone❗
• ❗var can shadow parameter even in strict mode❗️
• ❗var redeclaration applied even in strict mode❗️

📗 參考

• YDKJS: Scope & Closures (v.2) ⟩ Ch. 3: The Scope Chain

📘 手冊

• Statements and declarations ⟩ declaring variables ⟩ let