Revision 706251 of function expression

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 <div>
  {{jsSidebar("Operators")}}</div>
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<h2 id="Summary">Summary</h2>
<p>The <code>function</code> keyword can be used to define a function inside an expression.</p>
<h2 id="Syntax" name="Syntax">Syntax</h2>
<pre class="syntaxbox">
function [<em>name</em>]([<em>param1</em>[, <em>param2[</em>, ..., <em>paramN</em>]]]) {
   <em>statements</em>
}</pre>
<h3 id="Parameters" name="Parameters">Parameters</h3>
<dl>
 <dt>
  <code>name</code></dt>
 <dd>
  The function name. Can be omitted, in which case the function is <em>anonymous</em>. The name is only local to the function body.</dd>
 <dt>
  <code>paramN</code></dt>
 <dd>
  The name of an argument to be passed to the function. A function can have up to 255 arguments.</dd>
 <dt>
  <code>statements</code></dt>
 <dd>
  The statements which comprise the body of the function.</dd>
</dl>
<h2 id="Description" name="Description">Description</h2>
<p>A function expression is very similar to and has almost the same syntax as a function statement (see <a href="/en-US/docs/Web/JavaScript/Reference/Statements/function">function statement</a> for details). The main difference between a function expression and a function statement is the <em>function name,</em> which can be omitted in function expressions to create <em>anonymous</em> functions. See also <a href="/en-US/docs/Web/JavaScript/Reference/Functions_and_function_scope">functions and function scope</a> for more information.</p>
<h2 id="Examples" name="Examples">Examples</h2>
<p>The following example defines an unnamed function and assigns it to <code>x</code>. The function returns the square of its argument:</p>
<pre class="brush: js">
var x = function(y) {
   return y * y;
};
</pre>
<h3 id="Named_function_expression">Named function expression</h3>
<p>If you want to refer to the current function inside the function body, you need to create a named function expression. This name is then local only to the function body (scope). This also avoids using the non-standard <a href="/en-US/docs/Web/JavaScript/Reference/Functions_and_function_scope/arguments/callee">arguments.callee</a> property.</p>
<pre class="brush: js">
var math = {
  'factorial': function factorial(n) {
    if (n &lt;= 1)
      return 1;
    return n * factorial(n - 1);
  }
};
</pre>
<h2 id="Specifications">Specifications</h2>
<table class="standard-table">
 <tbody>
  <tr>
   <th scope="col">Specification</th>
   <th scope="col">Status</th>
   <th scope="col">Comment</th>
  </tr>
  <tr>
   <td>ECMAScript 3rd Edition.</td>
   <td>Standard</td>
   <td>Initial definition. Implemented in JavaScript 1.5</td>
  </tr>
  <tr>
   <td>{{SpecName('ES5.1', '#sec-13', 'Function definition')}}</td>
   <td>{{Spec2('ES5.1')}}</td>
   <td>&nbsp;</td>
  </tr>
  <tr>
   <td>{{SpecName('ES6', '#sec-function-definitions', 'Function definitions')}}</td>
   <td>{{Spec2('ES6')}}</td>
   <td>&nbsp;</td>
  </tr>
 </tbody>
</table>
<h2 id="Browser_compatibility">Browser compatibility</h2>
<p>{{ CompatibilityTable() }}</p>
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 <table class="compat-table">
  <tbody>
   <tr>
    <th>Feature</th>
    <th>Chrome</th>
    <th>Firefox (Gecko)</th>
    <th>Internet Explorer</th>
    <th>Opera</th>
    <th>Safari</th>
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   <tr>
    <td>Basic support</td>
    <td>{{ CompatVersionUnknown() }}</td>
    <td>{{ CompatVersionUnknown() }}</td>
    <td>{{ CompatVersionUnknown() }}</td>
    <td>{{ CompatVersionUnknown() }}</td>
    <td>{{ CompatVersionUnknown() }}</td>
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<div id="compat-mobile">
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    <th>Feature</th>
    <th>Android</th>
    <th>Chrome for Android</th>
    <th>Firefox Mobile (Gecko)</th>
    <th>IE Mobile</th>
    <th>Opera Mobile</th>
    <th>Safari Mobile</th>
   </tr>
   <tr>
    <td>Basic support</td>
    <td>{{ CompatVersionUnknown() }}</td>
    <td>{{ CompatVersionUnknown() }}</td>
    <td>{{ CompatVersionUnknown() }}</td>
    <td>{{ CompatVersionUnknown() }}</td>
    <td>{{ CompatVersionUnknown() }}</td>
    <td>{{ CompatVersionUnknown() }}</td>
   </tr>
  </tbody>
 </table>
</div>
<h2 id="See_also" name="See_also">See also</h2>
<ul>
 <li>{{jsxref("Functions_and_function_scope", "Functions and function scope")}}</li>
 <li>{{jsxref("Function")}}</li>
 <li>{{jsxref("Statements/function", "function statement")}}</li>
 <li>{{jsxref("Statements/function*", "function* statement")}}</li>
 <li>{{jsxref("Operators/function*", "function* expression")}}</li>
 <li>{{jsxref("GeneratorFunction")}}</li>
</ul>