在函数退出时，调用对象就会从作用域链中去掉。
When the function exits, the call object is removed from the scope chain.
全局变量总是最慢的，因为它们总是位于作用域链的最后一环。
Global variables are always the slowest to access because they are always last in the scope chain.
局部变量比域外变量快，因为它位于作用域链的第一个对象中。
Local variables are faster to access than out-of-scope variables because they exist in the first variable object of the scope chain.
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