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                ## undefined 與 undeclared 的區別? 已在作用域中聲明但還沒有賦值的變量,是 `undefined` 的。相反,還沒有在作用域中聲明過的變量,是 `undeclared` 的。 對于 `undeclared` 變量的引用,瀏覽器會報引用錯誤,如 `ReferenceError: b is not defined `。但是我們可以使用 `typeof` 的安全防范機制來避免報錯,因為對于 `undeclared`(或者 `not defined` )變量,`typeof` 會返回 `"undefined"`。
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