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）中存取。
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';
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