Revision 1069514 of SyntaxError: redeclaration of formal parameter "x"

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

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

<pre class="syntaxbox">
SyntaxError: redeclaration of formal parameter "x" (Firefox)
SyntaxError: Identifier "x" has already been declared (Chrome)
</pre>

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

<p>{{jsxref("SyntaxError")}}</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/let">let</a></code> assignment in a function body again. Redeclaring the same variable within the same function or block scope using <code>let</code> is not allowed in JavaScript.</p>

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

<p>In this case, the variable "arg" redeclares the argument.</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>Compatibility notes</h2>

<ul>
 <li>Prior to Firefox 49 {{geckoRelease(49)}}, this was thrown as a {{jsxref("TypeError")}} ({{bug(1275240)}}).</li>
</ul>

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