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                * let 和 const 定義的變量不會出現變量提升,而 var 定義的變量會提升。 * let 和 const 是JS中的塊級作用域 * let 和 const 不允許重復聲明(會拋出錯誤) * let 和 const 定義的變量在定義語句之前,如果使用會拋出錯誤(形成了暫時性死區),而 var 不會。 * const 聲明一個只讀的常量。一旦聲明,常量的值就不能改變(如果聲明是一個對象,那么不能改變的是對象的引用地址)
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