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+---
+title: function expression
+slug: Web/JavaScript/Reference/Operators/function
+translation_of: Web/JavaScript/Reference/Operators/function
+---
+<div>{{jsSidebar("Operators")}}</div>
+
+<p>Từ khóa <strong><code>function </code></strong><code>(hàm)</code> có thể dùng để định nghĩa chức năng bên trong một biểu thức.</p>
+
+<h2 id="Cú_pháp">Cú pháp</h2>
+
+<pre class="syntaxbox">var myFunction = function [<em>name</em>]([<em>param1</em>[, <em>param2[</em>, ..., <em>paramN</em>]]]) {
+   <em>statements</em>
+};</pre>
+
+<h3 id="Tham_số">Tham số</h3>
+
+<dl>
+ <dt><code>name</code></dt>
+ <dd>Tên hàm. Có thể bỏ qua, trong trường hợp chức năng đó là <code>vô danh. </code>Nó mô tả chính xác nhiệm vụ mà <strong>hàm </strong>sẽ làm.</dd>
+ <dt><code>paramN</code></dt>
+ <dd>Tên các đối số truyền vào.</dd>
+ <dt><code>statements</code></dt>
+ <dd>Các câu lệnh xử lý các đối số <strong><code>paramN </code></strong>truyền vào.</dd>
+</dl>
+
+<h2 id="Mô_tả">Mô tả</h2>
+
+<p>Một biểu thức chức năng (<code>function</code>) 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. A function expression can be used as a <strong>IIFE </strong>(Immediately Invoked Function Expression) which runs as soon as it is defined. See also the chapter about <a href="/en-US/docs/Web/JavaScript/Reference/Functions">functions</a> for more information.</p>
+
+<h3 id="Function_expression_hoisting">Function expression hoisting</h3>
+
+<p>Function expressions in JavaScript are not hoisted, unlike {{jsxref("Statements/function", "function declarations", "#Function_declaration_hoisting")}}. You can't use function expressions before you define them:</p>
+
+<pre class="brush: js">notHoisted(); // TypeError: notHoisted is not a function
+
+var notHoisted = function() {
+   console.log('bar');
+};
+</pre>
+
+<h2 id="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 <code><a href="/en-US/docs/Web/JavaScript/Reference/Functions/arguments/callee">arguments.callee</a></code> 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>{{SpecName('ESDraft', '#sec-function-definitions', 'Function definitions')}}</td>
+   <td>{{Spec2('ESDraft')}}</td>
+   <td> </td>
+  </tr>
+  <tr>
+   <td>{{SpecName('ES6', '#sec-function-definitions', 'Function definitions')}}</td>
+   <td>{{Spec2('ES6')}}</td>
+   <td> </td>
+  </tr>
+  <tr>
+   <td>{{SpecName('ES5.1', '#sec-13', 'Function definition')}}</td>
+   <td>{{Spec2('ES5.1')}}</td>
+   <td> </td>
+  </tr>
+  <tr>
+   <td>{{SpecName('ES3', '#sec-13', 'Function definition')}}</td>
+   <td>{{Spec2('ES3')}}</td>
+   <td>Initial definition. Implemented in JavaScript 1.5.</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>
+  <tr>
+   <td>Trailing comma in parameters</td>
+   <td>{{CompatUnknown}}</td>
+   <td>{{CompatGeckoDesktop("52.0")}}</td>
+   <td>{{CompatUnknown}}</td>
+   <td>{{CompatUnknown}}</td>
+   <td>{{CompatUnknown}}</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>
+  <tr>
+   <td>Trailing comma in parameters</td>
+   <td>{{CompatUnknown}}</td>
+   <td>{{CompatUnknown}}</td>
+   <td>{{CompatGeckoMobile("52.0")}}</td>
+   <td>{{CompatUnknown}}</td>
+   <td>{{CompatUnknown}}</td>
+   <td>{{CompatUnknown}}</td>
+   <td>{{CompatUnknown}}</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("Statements/function", "function statement")}}</li>
+ <li>{{jsxref("Statements/function*", "function* statement")}}</li>
+ <li>{{jsxref("Operators/function*", "function* expression")}}</li>
+ <li>{{jsxref("GeneratorFunction")}}</li>
+</ul>