有多种原因

1. 'const`变量将防止该值意外更改，例如如果您编写了 closure ，其中包含在直接作用域之外可以访问的变量.

1. A 'const` variable will prevent the value from accidentally changing, e.g. if you have written a closure that includes a variable that is accessible outside immediate scope.

JavaScript引擎尚不能常见的子表达式消除，这是一种非常常见的编译器优化.使用临时变量可以提高性能，例如在此示例中.

Javascript engines are not yet capable of common subexpression elimination, which is a very common compiler optimization. Using a temporary variable could improve performance, like in this example.

有时，使用不同名称的变量可以阐明功能；请参见编写真正明显的代码(ROC).示例:

Sometimes a variable with a different name can clarify the functionality; see Writing Really Obvious Code (ROC). Example:

var activeUsername = User.name;  //It's not just a user's name, it's the active user's name!