✅let redeclaration not allowed even in sloppy mode❗️

🚧 under construction

JS ⟩ declaration ⟩ variable ⟩ let ⟩ redeclaration not allowed

let doesn't allow redeclarations (even in sloppy mode)❗

⭐️ 重點

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❗

👥 相關

• 🔰let redeclaration

• ❗var redeclaration applied even in strict mode❗️

💈範例

• replit：let doesn't allow redeclarations

• 👉 ⛔️ SyntaxError ⟩ identifier 'xxx' has already been declared❗️

// ❌ function -> let ?
function f() { }
let f = 123;
//  ^
// ⛔ SyntaxError: Identifier 'f' has already been declared

// ❌ let -> let ?
let male = true;
let male = false;
//  ^^^^
// ⛔ SyntaxError: Identifier 'male' has already been declared

// ❌ var -> let ?
var name = 'Joe';
let name = 'Suzy';
//  ^^^^
// ⛔ SyntaxError: Identifier 'name' has already been declared

// ❌ let -> var ?
let age = 50;
var age = 25;
//  ^^^
// ⛔ SyntaxError: Identifier 'age' has already been declared

// ⭐️ block scope
{
    let x = 10;
    var x = 20;    // ❗ 這怪啦？ "var" 不是沒有 "block scope" 嗎？
    //  ^
    // ⛔ SyntaxError: Identifier 'x' has already been declared
}

📗 參考

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

🚧

• redeclaration