Revision 845193 of function

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

<p>The <strong>function declaration</strong> defines a function with the specified parameters.</p>

<p>You can also define functions using the {{jsxref("Function")}} constructor and a {{jsxref("Operators/function", "function expression")}}.</p>

<h2 id="Syntax">Syntax</h2>

<pre class="syntaxbox">
function <em>name</em>([<em>param</em>,[, <em>param</em>,[..., <em>param</em>]]]) {
   [<em>statements</em>]
}
</pre>

<dl>
 <dt><code>name</code></dt>
 <dd>The function name.</dd>
</dl>

<dl>
 <dt><code>param</code></dt>
 <dd>The name of an argument to be passed to the function. A function can have up to 255 arguments.</dd>
</dl>

<dl>
 <dt><code>statements</code></dt>
 <dd>The statements which comprise the body of the function.</dd>
</dl>

<h2 id="Description">Description</h2>

<p>A function created with a function declaration is a <code>Function</code> object and has all the properties, methods and behavior of <code>Function</code> objects. See {{jsxref("Function")}} for detailed information on functions.</p>

<p>A function can also be created using an expression (see {{jsxref("Operators/function", "function expression")}}).</p>

<p>By default, functions return <code>undefined</code>. To return any other value, the function must have a {{jsxref("Statements/return", "return")}} statement that specifies the value to return.</p>

<h3 id="Conditionally_created_functions">Conditionally created functions</h3>

<p>Functions can be conditionally declared, that is, a function statement can be nested within an <code>if</code> statement. Most browsers other than Mozilla will treat such conditional declarations as an unconditional declaration and create the function whether the condition is true or not, see <a href="https://kangax.github.io/nfe/#function-statements">this article</a> for an overview. Therefore they should not be used, for conditional creation use function expressions.</p>

<h3 id="Function_declaration_hoisting">Function declaration hoisting</h3>

<p>Function declarations in JavaScript are hoisting the function definition. You can use the function before you declared it:</p>

<pre class="brush: js">
hoisted(); // logs "foo"

function hoisted() {
  console.log("foo");
}
</pre>

<p>Note that {{jsxref("Operators/function", "function expressions")}} are not hoisted:</p>

<pre class="brush: js">
notHoisted(); // TypeError: notHoisted is not a function

var notHoisted = function() {
   console.log("bar");
};
</pre>

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

<h3 id="Using_function">Using <code>function</code></h3>

<p>The following code declares a function that returns the total amount of sales, when given the number of units sold of products <code>a</code>, <code>b</code>, and <code>c</code>.</p>

<pre class="brush: js">
function calc_sales(units_a, units_b, units_c) {
   return units_a*79 + units_b * 129 + units_c * 699;
}
</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>{{SpecName('ES6', '#sec-function-definitions', 'Function definitions')}}</td>
   <td>{{Spec2('ES6')}}</td>
   <td>&nbsp;</td>
  </tr>
  <tr>
   <td>{{SpecName('ES5.1', '#sec-13', 'Function definition')}}</td>
   <td>{{Spec2('ES5.1')}}</td>
   <td>&nbsp;</td>
  </tr>
  <tr>
   <td>{{SpecName('ES3', '#sec-13', 'Function definition')}}</td>
   <td>{{Spec2('ES3')}}</td>
   <td>&nbsp;</td>
  </tr>
  <tr>
   <td>{{SpecName('ES1', '#sec-13', 'Function definition')}}</td>
   <td>{{Spec2('ES1')}}</td>
   <td>Initial definition. Implemented in JavaScript 1.0.</td>
  </tr>
 </tbody>
</table>

<h2 id="Browser_compatibility">Browser compatibility</h2>

<p>{{CompatibilityTable}}</p>

<div id="compat-desktop">
<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>
  </tr>
  <tr>
   <td>Basic support</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
  </tr>
 </tbody>
</table>
</div>

<div id="compat-mobile">
<table class="compat-table">
 <tbody>
  <tr>
   <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">See also</h2>

<ul>
 <li>{{jsxref("Functions_and_function_scope", "Functions and function scope")}}</li>
 <li>{{jsxref("Function")}}</li>
 <li>{{jsxref("Operators/function", "function expression")}}</li>
 <li>{{jsxref("Statements/function*", "function* statement")}}</li>
 <li>{{jsxref("Operators/function*", "function* expression")}}</li>
 <li>{{jsxref("Functions/Arrow_functions", "Arrow functions")}}</li>
 <li>{{jsxref("GeneratorFunction")}}</li>
</ul>