// ❌ 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
}