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+---
+title: 'SyntaxError: redeclaration of formal parameter "x"'
+slug: Web/JavaScript/Reference/Errors/Redeclared_parameter
+tags:
+  - Errors
+  - JavaScript
+  - SyntaxError
+translation_of: Web/JavaScript/Reference/Errors/Redeclared_parameter
+---
+<div>{{jsSidebar("Errors")}}</div>
+
+<h2 id="信息">信息</h2>
+
+<pre class="syntaxbox">SyntaxError: redeclaration of formal parameter "x" (Firefox)
+SyntaxError: Identifier "x" has already been declared (Chrome)
+SyntaxError: Cannot declare a let variable twice: 'x' (WebKit)
+</pre>
+
+<h2 id="错误类型">错误类型</h2>
+
+<p>{{jsxref("SyntaxError")}}</p>
+
+<h2 id="哪里出错了">哪里出错了?</h2>
+
+<p>某个变量名称已经作为函数参数出现了，但是又使用了 <code><a href="/en-US/docs/Web/JavaScript/Reference/Statements/let">let</a></code> 在函数体里重声明了。在JavaScript 中不允许使用let在相同的函数或块范围内重新声明相同的变量。</p>
+
+<h2 id="示例">示例</h2>
+
+<p>在本例中，参数 "arg" 又重新声明了：</p>
+
+<pre class="brush: js example-bad">function f(arg) {
+  let arg = "foo";
+}
+
+// SyntaxError: redeclaration of formal parameter "arg"
+</pre>
+
+<p>如果要更改函数体中的“arg”的值，可以像下面一样，但不需要再次声明同一个变量。 换句话说：你可以省略 let 关键字。 如果要创建一个新变量，则需要将其重命名，因为其与函数参数有冲突。</p>
+
+<pre class="brush: js example-good">function f(arg) {
+  arg = "foo";
+}
+
+function f(arg) {
+  let bar = "foo";
+}
+</pre>
+
+<h2 id="兼容性提醒">兼容性提醒</h2>
+
+<ul>
+ <li>在 Firefox 49 {{geckoRelease(49)}} 之前，会抛出的是 {{jsxref("TypeError")}} ({{bug(1275240)}})。</li>
+</ul>
+
+<h2 id="相关">相关</h2>
+
+<ul>
+ <li><code><a href="/en-US/docs/Web/JavaScript/Reference/Statements/let">let</a></code></li>
+ <li><code><a href="/en-US/docs/Web/JavaScript/Reference/Statements/const">const</a></code></li>
+ <li><code><a href="/en-US/docs/Web/JavaScript/Reference/Statements/var">var</a></code></li>
+ <li><a href="/en-US/docs/Web/JavaScript/Guide/Grammar_and_Types#Declarations">Declaring variables</a> in the <a href="/en-US/docs/Web/JavaScript/Guide">JavaScript Guide</a></li>
+</ul>