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                [TOC] ## **var和let的區別** 1.var 能**聲明提前**,能**重復定義**, 2.let具有**塊級作用域**,不能聲明提前,不能重復定義 3.const定義的常量**不可修改**,必須**初始化**。 ``` function f(){ var a = 1;//去掉var,則輸出不會出錯,會輸出1 } f(); console.log(a);//會報錯 ``` ``` { var a = 1; var b = 2; } console.log(a);//1 console.log(b);//報錯 ``` ## 為什么要有let 因為let 具有塊級作用域 #### 區別具體代碼: ``` <script> /*1. let具有塊級作用域 { var a = 10; let b = 11; } console.log(a); console.log(b) */ /* 2. console.log(a); console.log(b); var a = 10; let b = 11; */ /* 3.var 能重復定義,let不能 var a = 10; var a = 30; console.log(a) */ let b = 11; let b = 20; console.log(b); </script> ```
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