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                ```js // const k; // 報錯 const k = 100; // const k = 200; // 報錯 var i = 99; function test() { // console.log(i); // 報錯 let i = 10; console.log(i); } test() // 10 console.log(i) //99 ``` var定義的變量可以修改,如果不初始化會輸出undefined,不會報錯。 const定義的變量不可以修改,而且必須初始化。不然報錯。 let是塊級作用域,函數內部使用let定義后,對函數外部無影響。
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