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SyntaxError: redeclaration of formal parameter "x"

JavaScript 的「redeclaration of formal parameter」例外會在函式參數中出現了相同的變數名稱，且函式主體中又再次使用 let 賦值來重複宣告它時發生。

In this article

訊息

SyntaxError: Identifier "x" has already been declared（基於 V8） SyntaxError: redeclaration of formal parameter "x"（Firefox） SyntaxError: Cannot declare a let variable twice: 'x'.（Safari）

錯誤類型

SyntaxError

發生了什麼問題？

當函式參數中出現了相同的變數名稱，且函式主體中又再次使用 let 賦值來重複宣告它時，就會發生此錯誤。JavaScript 不允許在相同的函式或區塊作用域中，使用 let 重複宣告相同的變數。

範例

重複宣告的引數

在這個例子中，變數「arg」重複宣告了引數。

js
function f(arg) { let arg = "foo"; } // SyntaxError: redeclaration of formal parameter "arg"

如果你想在函式主體中改變「arg」的值，你可以這麼做，但你不需要再次宣告相同的變數。換句話說：你可以省略 let 關鍵字。如果你想建立一個新的變數，你需要將它重新命名，以避免與現有的函式參數衝突。

js
function f(arg) { arg = "foo"; } function g(arg) { let bar = "foo"; }

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    78. SyntaxError: redeclaration of formal parameter "x"
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