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+---
+title: 'SyntaxError: redeclaration of formal parameter "x"'
+slug: Web/JavaScript/Reference/Errors/Redeclared_parameter
+translation_of: Web/JavaScript/Reference/Errors/Redeclared_parameter
+---
+<div>{{jsSidebar("Errors")}}</div>
+
+<h2 id="訊息">訊息</h2>
+
+<pre class="syntaxbox">SyntaxError: redeclaration of formal parameter "x" (Firefox)
+SyntaxError: Identifier "x" has already been declared (Chrome)
+</pre>
+
+<h2 id="錯誤類型">錯誤類型</h2>
+
+<p>{{jsxref("SyntaxError")}}</p>
+
+<h2 id="哪裡錯了？">哪裡錯了？</h2>
+
+<p>當相同的變數名作為函式的參數、接著又在函式體（function body）內用了 <code><a href="/zh-TW/docs/Web/JavaScript/Reference/Statements/let">let</a></code> 重複宣告並指派時出現。在 JavaScript 裡面，不允許在相同的函式、或是作用域區塊（block scope）內重複宣告相同的 <code>let</code> 變數。</p>
+
+<h2 id="實例">實例</h2>
+
+<p>在這裡，「arg」變數的參數被重複宣告。</p>
+
+<pre class="brush: js example-bad">function f(arg) {
+  let arg = 'foo';
+}
+
+// SyntaxError: redeclaration of formal parameter "arg"
+</pre>
+
+<p>If you want to change the value of "arg" in the function body, you can do so, but you do not need to declare the same variable again. In other words: you can omit the <code>let</code> keyword. If you want to create a new variable, you need to rename it as conflicts with the function parameter already.</p>
+
+<pre class="brush: js example-good">function f(arg) {
+  arg = 'foo';
+}
+
+function f(arg) {
+  let bar = 'foo';
+}
+</pre>
+
+<h2 id="相容性註解">相容性註解</h2>
+
+<ul>
+ <li>在 Firefox 49 {{geckoRelease(49)}} 之前，這個錯誤被歸為 {{jsxref("TypeError")}}。 ({{bug(1275240)}})</li>
+</ul>
+
+<h2 id="參見">參見</h2>
+
+<ul>
+ <li><code><a href="/zh-TW/docs/Web/JavaScript/Reference/Statements/let">let</a></code></li>
+ <li><code><a href="/zh-TW/docs/Web/JavaScript/Reference/Statements/const">const</a></code></li>
+ <li><code><a href="/zh-TW/docs/Web/JavaScript/Reference/Statements/var">var</a></code></li>
+ <li>在 <a href="/zh-TW/docs/Web/JavaScript/Guide">JavaScript 教學</a>內<a href="/zh-TW/docs/Web/JavaScript/Guide/Grammar_and_Types#Declarations">宣告變數</a> </li>
+</ul>