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javascript中的变量作用域scope

 在function之外的变量，声明为const, var或let,则这个变量是全局可见的；在函数内声明的变量，如果变量不带const, var或let,则这个变量也是全局的。

在函数内部定义的或者参数变量，是局部变量，函数内可见。

可以同时定义全局变量和局部变量，而且变量名可以相同，此时局部变量优先级高于全局变量。

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