❗accessing var before declaration gets undefined❗️

JS ⟩ variable ⟩ var ⟩ access before declaration gets "undefined"

(unexpected result)

var has no temporal dead zone, but accessing it before its declaration will get undefined, instead of its initial value (if exists)❗

💈範例

• replit：access var before declaration

// ❗ "var" has no "temporal dead zone", 
//    but it's `undefined` initially.

// ⭐️ access before declaration  ╭── ⭐️ ───╮
log(aVar, typeof aVar);     // ❗ undefined, 'undefined'

// ┌─────────────────┐
// │ var declaration │
// └─────────────────┘

// ⭐️ access after declaration
var aVar = 'hello';

log(aVar, typeof aVar);     // ❗ "hello", 'undefined'

👥 相關

• ✅stop using var❗️

• ❗var hoisting - var is hoisted to top of enclosing function scope.

📘 手冊

• Statements and declarations ⟩ declaring variables ⟩ var