Revision 1061782 of TypeError: variable "x" redeclares argument

<div>{{jsSidebar("Errors")}}</div>

<h2 id="Message">Message</h2>

<pre class="syntaxbox">
TypeError: variable "x" redeclares argument (Firefox)
</pre>

<h2 id="Error_type">Error type</h2>

<p>{{jsxref("TypeError")}} warning in <a href="/en-US/docs/Web/JavaScript/Reference/Strict_mode">strict mode</a> only.</p>

<h2 id="What_went_wrong">What went wrong?</h2>

<p>The same variable name occurs as a function parameter and is then redeclared using a <code><a href="/en-US/docs/Web/JavaScript/Reference/Statements/var">var</a></code> assignment in a function body again. This might be a naming conflict and thus JavaScript warns about it.</p>

<p>This error occurs as a warning in <a href="/en-US/docs/Web/JavaScript/Reference/Strict_mode">strict mode code</a> only. In non-strict code, the redeclaration is silently ignored.</p>

<h2 id="Examples">Examples</h2>

<h3 id="Invalid_cases">Invalid cases</h3>

<p>In this case, the variable "arg" redeclares the argument.</p>

<pre class="brush: js example-bad">
"use strict";

function f(arg) { 
  var arg = "foo"; 
}
</pre>

<h3 id="Valid_cases">Valid cases</h3>

<p>To fix this warning, the <code><a href="/en-US/docs/Web/JavaScript/Reference/Statements/var">var</a></code> statement can just be omitted, because the variable exists already. In other cases, you might to rename either the function parameter or the variable name.</p>

<pre class="brush: js example-good">
"use strict";

function f(arg) {
  arg = "foo";
}
</pre>

<h2 id="See_also">See also</h2>

<ul>
 <li><a href="/en-US/docs/Web/JavaScript/Reference/Strict_mode">Strict mode</a></li>
</ul>