使用 var 關鍵字聲明的變數的作用域為創建它們的函數，或者如果在任何函數外部創建，則為全域物件。 let 和 const 是區塊作用域的，這意味著它們只能在最近的一組花括號（函數、if-else 區塊或 for 迴圈）內存取。

function foo() {
  // All variables are accessible within functions.
  var bar = 'bar';
  let baz = 'baz';
  const qux = 'qux';

  console.log(bar); // bar
  console.log(baz); // baz
  console.log(qux); // qux
}

console.log(bar); // ReferenceError: bar is not defined
console.log(baz); // ReferenceError: baz is not defined
console.log(qux); // ReferenceError: qux is not defined

if (true) {
  var bar = 'bar';
  let baz = 'baz';
  const qux = 'qux';
}

// var declared variables are accessible anywhere in the function scope.
console.log(bar); // bar
// let and const defined variables are not accessible outside the block they were defined in.
console.log(baz); // ReferenceError: baz is not defined
console.log(qux); // ReferenceError: qux is not defined

var 允許變數被提升，這意味著它們可以在聲明之前在程式碼中引用。 let 和 const 不允許這樣做，而是拋出錯誤。

console.log(foo); // undefined

var foo = 'foo';

console.log(baz); // ReferenceError: can't access lexical declaration 'baz' before initialization

let baz = 'baz';

console.log(bar); // ReferenceError: can't access lexical declaration 'bar' before initialization

const bar = 'bar';

用 var 重新宣告變數不會拋出錯誤，但 let 和 const 會拋出錯誤。

var foo = 'foo';
var foo = 'bar';
console.log(foo); // "bar"

let baz = 'baz';
let baz = 'qux'; // Uncaught SyntaxError: Identifier 'baz' has already been declared

let 和 const 的不同之處在於 let 允許重新分配變數的值，而 const 則不允許。

// This is fine.
let foo = 'foo';
foo = 'bar';

// This causes an exception.
const baz = 'baz';
baz = 'qux';