❗var in block can't shadow outer let❗️

🚧 under construction

JS ⟩ variable ⟩ shadowing ⟩ var in block can't shadow outer let

💈範例

• replit：var in block can't shadow outer let

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

function f() {
    
    let a = 1;       // let declaration
    
    // var a = 2;    // <----- var declaration is hoisted here❗️
                     //        which raises a redeclaration SyntaxError.
                     // (let doesn't allow redeclaration)
    
    // block scope
    {
        // ⭐️ `var` doesn't have block scope, 
        //     cannot shadow `let` variable in outer enclosing scope.
        var a = 2;
        //  ^
        // ⛔ SyntaxError: Identifier 'a' has already been declared
    }
    
    // another block
    {
        let b = 3;

        // ❗ although `var` doesn't have "block scope", 
        //    we can't put let/var in the same block either.
        var b = 4;
        //  ^
        // ⛔ SyntaxError: Identifier 'b' has already been declared
    }
    
}

👥 相關

• var has no block scope❗️❗
• one of the reasons of stop using var❗️

🗣 討論

• shadowing parameters by local variables? (my question)

📗 參考

• YDKJS: Scope & Closures (v.2) ⟩ Ch. 8 ⟩ Parameter Scope