❗var has no block scope❗️

JS ⟩ variable ⟩ var ⟩ has no block scope

var in block doesn't have block scope, cannot shadow let variable in the outer enclosing scope.

👥 相關

• variable shadowing

• var in block can't shadow outer let❗️❗

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

• let redeclaration not allowed even in sloppy mode❗️ - let doesn't allow redeclaration.

💈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
    }
    
}

📘 手冊

• Statements and declarations ⟩ declaring variables ⟩ var