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---
title: Expressão função (Function expression)
slug: Web/JavaScript/Reference/Operadores/função
tags:
- Expressões Primárias
- Funiconaldiade de Linguagem
- Função
- JavaScript
- Operador
translation_of: Web/JavaScript/Reference/Operators/function
---
<div>{{jsSidebar("Operators")}}</div>
<p>A palavra-chave <strong><code>function</code></strong> pode ser utilziada para definir uma função dentro de uma expressão.</p>
<p>You can also define functions using the <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function"><code>Function</code></a> constructor and a <a href="/en-US/docs/Web/JavaScript/Reference/Statements/function"><code>function declaration</code></a>.</p>
<div>{{EmbedInteractiveExample("pages/js/expressions-functionexpression.html", "shorter")}}</div>
<h2 id="Sintaxe">Sintaxe</h2>
<pre class="syntaxbox">let <var>myFunction</var> = function [<var>name</var>]([<var>param1</var>[, <var>param2[</var>, ..., <var>paramN</var>]]]) {
<var>statements</var>
};</pre>
<p>As of ES2015, you can also use {{jsxref("Functions/Arrow_functions", "arrow functions")}}.</p>
<h3 id="Parâmetros">Parâmetros</h3>
<dl>
<dt><code><var>name</var></code> {{optional_inline}}</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><var>paramN</var></code> {{optional_inline}}</dt>
<dd>The name of an argument to be passed to the function.</dd>
<dt><code><var>statements</var></code> {{optional_inline}}</dt>
<dd>The statements which comprise the body of the function.</dd>
</dl>
<h2 id="Descrição">Descrição</h2>
<p>A function expression is very similar to and has almost the same syntax as a function declaration (see {{jsxref("Statements/function", "function statement")}} for details). The main difference between a function expression and a function declaration 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 an <a href="/en-US/docs/Glossary/IIFE">IIFE (Immediately Invoked Function Expression)</a> which runs as soon as it is defined. See also the chapter about {{jsxref("Functions", "functions")}} 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">console.log(notHoisted) // undefined
// even though the variable name is hoisted, the definition isn't. so it's undefined.
notHoisted(); // TypeError: notHoisted is not a function
var notHoisted = function() {
console.log('bar');
};
</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. <u><strong>This name is then local only to the function body (scope)</strong></u>. This also avoids using the non-standard {{jsxref("Functions/arguments/callee", "arguments.callee")}} property.</p>
<pre class="brush: js">let math = {
'factit': function factorial(n) {
console.log(n)
if (n <= 1) {
return 1;
}
return n * factorial(n - 1);
}
};
math.factit(3) //3;2;1;
</pre>
<p>The variable the function expression is assigned to will have a <code>name</code> property. The name doesn't change if it's assigned to a different variable. If function name is omitted, it will be the variable name (implicit name). If function name is present, it will be the function name (explicit name). This also applies to {{jsxref("Functions/Arrow_functions", "arrow functions")}} (arrows don't have a name so you can only give the variable an implicit name).</p>
<pre class="brush: js">var foo = function() {}
foo.name // "foo"
var foo2 = foo
foo2.name // "foo"
var bar = function baz() {}
bar.name // "baz"
console.log(foo === foo2); // true
console.log(typeof baz); // undefined
console.log(bar === baz); // false (errors because baz == undefined)
</pre>
<h2 id="Exemplos">Exemplos</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>
<p>More commonly it is used as a <a href="/en-US/docs/Glossary/Callback_function">callback</a>:</p>
<pre class="brush: js">button.addEventListener('click', function(event) {
console.log('button is clicked!')
})</pre>
<h2 id="Especificações">Especificações</h2>
<table class="standard-table">
<thead>
<tr>
<th scope="col">Especificação</th>
</tr>
</thead>
<tbody>
<tr>
<td>{{SpecName('ESDraft', '#sec-function-definitions', 'Function definitions')}}</td>
</tr>
</tbody>
</table>
<h2 id="Compatibilidade_de_navegador">Compatibilidade de navegador</h2>
<p>{{Compat("javascript.operators.function")}}</p>
<h2 id="Consulte_também">Consulte também </h2>
<ul>
<li>{{jsxref("Arrow_functions", "Arrow functions")}}</li>
<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>
<li>{{jsxref("Statements/async_function", "async function")}}</li>
<li>{{jsxref("Operators/async_function", "async function expression")}}</li>
</ul>
|
