path: root/files/pt-pt/web/javascript/reference/functions/index.html

---
title: Funções
slug: Web/JavaScript/Reference/Functions
tags:
  - Constructor
  - Função
  - Funções
  - JavaScript
  - Parâmetro
  - parâmetros
translation_of: Web/JavaScript/Reference/Functions
original_slug: Web/JavaScript/Reference/Funcoes
---
<div>{{jsSidebar("Functions")}}</div>

<p>Generally speaking, a function is a "subprogram" that can be <em>called</em> by code external (or internal in the case of recursion) to the function. Like the program itself, a function is composed of a sequence of statements called the <em>function body</em>. Values can be <em>passed</em> to a function, and the function will <em>return</em> a value.</p>

<p>In JavaScript, functions are first-class objects, because they can have properties and methods just like any other object. What distinguishes them from other objects is that functions can be called. In brief, they are <code><a href="/en-US/docs/JavaScript/Reference/Global_Objects/Function">Function</a></code> objects.</p>

<p>For more examples and explanations, see also the <a href="/en-US/docs/Web/JavaScript/Guide/Functions">JavaScript guide about functions</a>.</p>

<h2 id="Descrição">Descrição</h2>

<p>Every function in JavaScript is a <code>Function</code> object. See {{jsxref("Function")}} for information on properties and methods of <code>Function</code> objects.</p>

<p>To return a value other than the default, a function must have a <code><a href="/en-US/docs/Web/JavaScript/Reference/Statements/return">return</a></code> statement that specifies the value to return. A function without a return statement will return a default value. In the case of a <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/constructor">constructor</a> called with the <code><a href="/en-US/docs/Web/JavaScript/Reference/Operators/new">new</a></code> keyword, the default value is the value of its <code>this</code> parameter. For all other functions, the default return value is {{jsxref("undefined")}}.</p>

<p>The parameters of a function call are the function's <em>arguments</em>. Arguments are passed to functions <em>by value</em>. If the function changes the value of an argument, this change is not reflected globally or in the calling function. However, object references are values, too, and they are special: if the function changes the referred object's properties, that change is visible outside the function, as shown in the following example:</p>

<pre class="brush: js">/* Declare the function 'myFunc' */
function myFunc(theObject) {
  theObject.brand = "Toyota";
}

/*
 * Declare variable 'mycar';
 * create and initialize a new Object;
 * assign reference to it to 'mycar'
 */
var mycar = {
  brand: "Honda",
  model: "Accord",
  year: 1998
};

/* Logs 'Honda' */
console.log(mycar.brand);

/* Pass object reference to the function */
myFunc(mycar);

/*
 * Logs 'Toyota' as the value of the 'brand' property
 * of the object, as changed to by the function.
 */
console.log(mycar.brand);
</pre>

<p>The <a href="/en-US/docs/Web/JavaScript/Reference/Operators/this"><code>this</code> keyword</a> does not refer to the currently executing function, so you must refer to <code>Function</code> objects by name, even within the function body.</p>

<h2 id="Definindo_funções">Definindo funções</h2>

<p>There are several ways to define functions:</p>

<h3 id="The_function_declaration_(function_statement)">The function declaration (<code>function</code> statement)</h3>

<p>There is a special syntax for declaring functions (see <a href="/en-US/docs/Web/JavaScript/Reference/Statements/function">function statement</a> for details):</p>

<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 comprising the body of the function.</dd>
</dl>

<h3 id="The_function_expression_(function_expression)">The function expression (<code>function</code> expression)</h3>

<p>A function expression is similar to and has the same syntax as a function declaration (see <a href="/en-US/docs/Web/JavaScript/Reference/Operators/function">function expression</a> for details). A function expression may be a part of a larger expression. One can define "named" function expressions (where the name of the expression might be used in the call stack for example) or "anonymous" function expressions. Function expressions are not <em>hoisted</em> onto the beginning of the scope, therefore they cannot be used before they appear in the code.</p>

<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. Can be omitted, in which case the function becomes known as an anonymous function.</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>
 <dt><code>statements</code></dt>
 <dd>The statements comprising the body of the function.</dd>
</dl>

<p>Here is an example of an <strong>anonymous</strong> function expression (the <code>name</code> is not used):</p>

<pre class="brush: js">var myFunction = function() {
    statements
}</pre>

<p>It is also possible to provide a name inside the definition in order to create a <strong>named</strong> function expression:</p>

<pre class="brush: js">var myFunction = function namedFunction(){
    statements
}
</pre>

<p>One of the benefit of creating a named function expression is that in case we encounted an error, the stack trace will contain the name of the function, making it easier to find the origin of the error.</p>

<p>As we can see, both examples do not start with the <code>function</code> keyword. Statements involving functions which do not start with <code>function</code> are function expressions.</p>

<p>When functions are used only once, a common pattern is an <strong>IIFE (<em>Immediately Invokable Function Expression</em>)</strong>.</p>

<pre class="brush: js">(function() {
    statements
})();</pre>

<p>IIFE are function expressions that are invoked as soon as the function is declared.</p>

<h3 id="The_generator_function_declaration_(function*_statement)">The generator function declaration (<code>function*</code> statement)</h3>

<p>There is a special syntax for generator function declarations (see {{jsxref('Statements/function*', 'function* statement')}} for details):</p>

<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 comprising the body of the function.</dd>
</dl>

<h3 id="The_generator_function_expression_(function*_expression)">The generator function expression (<code>function*</code> expression)</h3>

<p>A generator function expression is similar to and has the same syntax as a generator function declaration (see {{jsxref('Operators/function*', 'function* expression')}} for details):</p>

<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. Can be omitted, in which case the function becomes known as an anonymous function.</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>
 <dt><code>statements</code></dt>
 <dd>The statements comprising the body of the function.</dd>
</dl>

<h3 id="The_arrow_function_expression_(>)">The arrow function expression (=&gt;)</h3>

<p>An arrow function expression has a shorter syntax and lexically binds its <code>this</code> value (see <a href="/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions">arrow functions</a> for details):</p>

<pre class="syntaxbox">([param[, param]]) =&gt; {
   statements
}

param =&gt; expression
</pre>

<dl>
 <dt><code>param</code></dt>
 <dd>The name of an argument. Zero arguments need to be indicated with <code>()</code>.  For only one argument, the parentheses are not required. (like <code>foo =&gt; 1</code>)</dd>
 <dt><code>statements or expression</code></dt>
 <dd>Multiple statements need to be enclosed in brackets. A single expression requires no brackets. The expression is also the implicit return value of the function.</dd>
</dl>

<h3 id="The_Function_constructor">The <code>Function</code> constructor</h3>

<div class="note">
<p><strong>Note:</strong> Using the <code>Function</code> constructor to create functions is not recommended since it needs the function body as a string which may prevent some JS engine optimizations and can also cause other problems.</p>
</div>

<p>As all other objects, {{jsxref("Function")}} objects can be created using the <code>new</code> operator:</p>

<pre class="syntaxbox">new Function (<em>arg1</em>, <em>arg2</em>, ... <em>argN</em>, <em>functionBody</em>)
</pre>

<dl>
 <dt><code>arg1, arg2, ... arg<em>N</em></code></dt>
 <dd>Zero or more names to be used by the function as formal parameters. Each must be a proper JavaScript identifier.</dd>
</dl>

<dl>
 <dt><code>functionBody</code></dt>
 <dd>A string containing the JavaScript statements comprising the function body.</dd>
</dl>

<p>Invoking the <code>Function</code> constructor as a function (without using the <code>new</code> operator) has the same effect as invoking it as a constructor.</p>

<h3 id="The_GeneratorFunction_constructor">The <code>GeneratorFunction</code> constructor</h3>

<div class="note">
<p><strong>Note:</strong> <code>GeneratorFunction</code> is not a global object, but could be obtained from generator function instance (see {{jsxref("GeneratorFunction")}} for more detail).</p>
</div>

<div class="note">
<p><strong>Note:</strong> Using the <code>GeneratorFunction</code> constructor to create functions is not recommended since it needs the function body as a string which may prevent some JS engine optimizations and can also cause other problems.</p>
</div>

<p>As all other objects, {{jsxref("GeneratorFunction")}} objects can be created using the <code>new</code> operator:</p>

<pre class="syntaxbox">new GeneratorFunction (<em>arg1</em>, <em>arg2</em>, ... <em>argN</em>, <em>functionBody</em>)
</pre>

<dl>
 <dt><code>arg1, arg2, ... arg<em>N</em></code></dt>
 <dd>Zero or more names to be used by the function as formal argument names. Each must be a string that conforms to the rules for a valid JavaScript identifier or a list of such strings separated with a comma; for example "<code>x</code>", "<code>theValue</code>", or "<code>a,b</code>".</dd>
</dl>

<dl>
 <dt><code>functionBody</code></dt>
 <dd>A string containing the JavaScript statements comprising the function definition.</dd>
</dl>

<p>Invoking the <code>Function</code> constructor as a function (without using the <code>new</code> operator) has the same effect as invoking it as a constructor.</p>

<h2 id="Parâmetros_de_função">Parâmetros de função</h2>

<h3 id="Parâmetros_predefinidos">Parâmetros predefinidos</h3>

<p>Default function parameters allow formal parameters to be initialized with default values if no value or <code>undefined</code> is passed. For more details, see<a href="/en-US/docs/Web/JavaScript/Reference/Functions/Default_parameters"> default parameters</a>.</p>

<h3 id="Parâmetros_Rest">Parâmetros <em>Rest</em></h3>

<p>The rest parameter syntax allows to represent an indefinite number of arguments as an array. For more details, see <a href="/en-US/docs/Web/JavaScript/Reference/Functions/rest_parameters">rest parameters</a>.</p>

<h2 id="O_objeto_arguments">O objeto <em><code>arguments</code></em></h2>

<p>You can refer to a function's arguments within the function by using the <code>arguments</code> object. See <a href="/en-US/docs/Web/JavaScript/Reference/Functions/arguments">arguments</a>.</p>

<ul>
 <li><code><a href="/en-US/docs/JavaScript/Reference/Functions_and_function_scope/arguments">arguments</a></code>: An array-like object containing the arguments passed to the currently executing function.</li>
 <li><code><a href="/en-US/docs/JavaScript/Reference/Functions_and_function_scope/arguments/callee">arguments.callee</a></code> {{Deprecated_inline}}: The currently executing function.</li>
 <li><code><a href="/en-US/docs/JavaScript/Reference/Functions_and_function_scope/arguments/caller">arguments.caller</a></code> {{Obsolete_inline}} : The function that invoked the currently executing function.</li>
 <li><code><a href="/en-US/docs/JavaScript/Reference/Functions_and_function_scope/arguments/length">arguments.length</a></code>: The number of arguments passed to the function.</li>
</ul>

<h2 id="Definindo_funções_de_método">Definindo funções de método</h2>

<h3 id="Getter_and_setter_functions">Getter and setter functions</h3>

<p>You can define getters (accessor methods) and setters (mutator methods) on any standard built-in object or user-defined object that supports the addition of new properties. The syntax for defining getters and setters uses the object literal syntax.</p>

<dl>
 <dt><a href="/en-US/docs/Web/JavaScript/Reference/Functions/get">get</a></dt>
 <dd>
 <p>Binds an object property to a function that will be called when that property is looked up.</p>
 </dd>
 <dt><a href="/en-US/docs/Web/JavaScript/Reference/Functions/set">set</a></dt>
 <dd>Binds an object property to a function to be called when there is an attempt to set that property.</dd>
</dl>

<h3 id="Method_definition_syntax">Method definition syntax</h3>

<p>Starting with ECMAScript 2015, you are able to define own methods in a shorter syntax, similar to the getters and setters. See <a href="/en-US/docs/Web/JavaScript/Reference/Functions/Method_definitions">method definitions</a> for more information.</p>

<pre class="brush: js">var obj = {
  foo() {},
  bar() {}
};</pre>

<h2 id="Constructor_vs._declaration_vs._expression">Constructor vs. declaration vs. expression</h2>

<p>Compare the following:</p>

<p>A function defined with the <code>Function</code> <em>constructor</em> assigned to the variable <code>multiply:</code></p>

<pre class="brush: js">var multiply = new Function('x', 'y', 'return x * y');</pre>

<p>A <em>function declaration</em> of a function named <code>multiply</code>:</p>

<pre class="brush: js">function multiply(x, y) {
   return x * y;
} // there is no semicolon here
</pre>

<p>A <em>function expression</em> of an anonymous function assigned to the variable <code>multiply:</code></p>

<pre class="brush: js">var multiply = function(x, y) {
   return x * y;
};
</pre>

<p>A <em>function expression</em> of a function named <code>func_name</code> assigned to the variable <code>multiply:</code></p>

<pre class="brush: js">var multiply = function func_name(x, y) {
   return x * y;
};
</pre>

<h3 id="Differences">Differences</h3>

<p>All do approximately the same thing, with a few subtle differences:</p>

<p>There is a distinction between the function name and the variable the function is assigned to. The function name cannot be changed, while the variable the function is assigned to can be reassigned. The function name can be used only within the function's body. Attempting to use it outside the function's body results in an error (or <code>undefined</code> if the function name was previously declared via a <code>var</code> statement). For example:</p>

<pre class="brush: js">var y = function x() {};
alert(x); // throws an error
</pre>

<p>The function name also appears when the function is serialized via <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/toString"><code>Function</code>'s toString method</a>.</p>

<p>On the other hand, the variable the function is assigned to is limited only by its scope, which is guaranteed to include the scope in which the function is declared.</p>

<p>As the 4th example shows, the function name can be different from the variable the function is assigned to. They have no relation to each other. A function declaration also creates a variable with the same name as the function name. Thus, unlike those defined by function expressions, functions defined by function declarations can be accessed by their name in the scope they were defined in:</p>

<p>A function defined by '<code>new Function'</code> does not have a function name. However, in the <a href="/en-US/docs/Mozilla/Projects/SpiderMonkey">SpiderMonkey</a> JavaScript engine, the serialized form of the function shows as if it has the name "anonymous". For example, <code>alert(new Function())</code> outputs:</p>

<pre class="brush: js">function anonymous() {
}
</pre>

<p>Since the function actually does not have a name, <code>anonymous</code> is not a variable that can be accessed within the function. For example, the following would result in an error:</p>

<pre class="brush: js">var foo = new Function("alert(anonymous);");
foo();
</pre>

<p>Unlike functions defined by function expressions or by the <code>Function</code> constructor, a function defined by a function declaration can be used before the function declaration itself. For example:</p>

<pre class="brush: js">foo(); // alerts FOO!
function foo() {
   alert('FOO!');
}
</pre>

<p>A function defined by a function expression or by a function declaration inherits the current scope. That is, the function forms a closure. On the other hand, a function defined by a <code>Function</code> constructor does not inherit any scope other than the global scope (which all functions inherit).</p>

<pre class="brush: js">/*
 * Declare and initialize a variable 'p' (global)
 * and a function 'myFunc' (to change the scope) inside which
 * declare a varible with same name 'p' (current) and
 * define three functions using three different ways:-
 *     1. function declaration
 *     2. function expression
 *     3. function constructor
 * each of which will log 'p'
 */
var p = 5;
function myFunc() {
    var p = 9;

    function decl() {
        console.log(p);
    }
    var expr = function() {
        console.log(p);
    };
    var cons = new Function('\tconsole.log(p);');

    decl();
    expr();
    cons();
}
myFunc();

/*
 * Logs:-
 * 9  - for 'decl' by function declaration (current scope)
 * 9  - for 'expr' by function expression (current scope)
 * 5  - for 'cons' by Function constructor (global scope)
 */
</pre>

<p>Functions defined by function expressions and function declarations are parsed only once, while those defined by the <code>Function</code> constructor are not. That is, the function body string passed to the <code>Function</code> constructor must be parsed each and every time the constructor is called. Although a function expression creates a closure every time, the function body is not reparsed, so function expressions are still faster than "<code>new Function(...)</code>". Therefore the <code>Function</code> constructor should generally be avoided whenever possible.</p>

<p>It should be noted, however, that function expressions and function declarations nested within the function generated by parsing a <code>Function constructor</code> 's string aren't parsed repeatedly. For example:</p>

<pre class="brush: js">var foo = (new Function("var bar = \'FOO!\';\nreturn(function() {\n\talert(bar);\n});"))();
foo(); // The segment "function() {\n\talert(bar);\n}" of the function body string is not re-parsed.</pre>

<p>A function declaration is very easily (and often unintentionally) turned into a function expression. A function declaration ceases to be one when it either:</p>

<ul>
 <li>becomes part of an expression</li>
 <li>is no longer a "source element" of a function or the script itself. A "source element" is a non-nested statement in the script or a function body:</li>
</ul>

<pre class="brush: js">var x = 0;               // source element
if (x === 0) {           // source element
   x = 10;               // not a source element
   function boo() {}     // not a source element
}
function foo() {         // source element
   var y = 20;           // source element
   function bar() {}     // source element
   while (y === 10) {    // source element
      function blah() {} // not a source element
      y++;               // not a source element
   }
}
</pre>

<h3 id="Examples">Examples</h3>

<pre class="brush: js">// function declaration
function foo() {}

// function expression
(function bar() {})

// function expression
x = function hello() {}


if (x) {
   // function expression
   function world() {}
}


// function declaration
function a() {
   // function declaration
   function b() {}
   if (0) {
      // function expression
      function c() {}
   }
}
</pre>

<h2 id="Block-level_functions">Block-level functions</h2>

<p>In <a href="/en-US/docs/Web/JavaScript/Reference/Strict_mode">strict mode</a>, starting with ES2015, functions inside blocks are now scoped to that block. Prior to ES2015, block-level functions were forbidden in strict mode.</p>

<pre class="brush: js">'use strict';

function f() {
  return 1;
}

{
  function f() {
    return 2;
  }
}

f() === 1; // true

// f() === 2 in non-strict mode
</pre>

<h3 id="Block-level_functions_in_non-strict_code">Block-level functions in non-strict code</h3>

<p>In a word: Don't.</p>

<p>In non-strict code, function declarations inside blocks behave strangely. For example:</p>

<pre class="brush: js">if (shouldDefineZero) {
   function zero() {     // DANGER: compatibility risk
      console.log("This is zero.");
   }
}
</pre>

<p>ES2015 says that if <code>shouldDefineZero</code> is false, then <code>zero</code> should never be defined, since the block never executes. However, it's a new part of the standard. Historically, this was left unspecified, and some browsers would define <code>zero</code> whether the block executed or not.</p>

<p>In <a href="/en-US/docs/Web/JavaScript/Reference/Strict_mode">strict mode</a>, all browsers that support ES2015 handle this the same way: <code>zero</code> is defined only if <code>shouldDefineZero</code> is true, and only in the scope of the <code>if</code>-block.</p>

<p>A safer way to define functions conditionally is to assign a function expression to a variable:</p>

<pre class="brush: js">var zero;
if (shouldDefineZero) {
   zero = function() {
      console.log("This is zero.");
   };
}
</pre>

<h2 id="Exemplos">Exemplos</h2>

<h3 id="Returning_a_formatted_number">Returning a formatted number</h3>

<p>The following function returns a string containing the formatted representation of a number padded with leading zeros.</p>

<pre class="brush: js">// This function returns a string padded with leading zeros
function padZeros(num, totalLen) {
   var numStr = num.toString();             // Initialize return value as string
   var numZeros = totalLen - numStr.length; // Calculate no. of zeros
   for (var i = 1; i &lt;= numZeros; i++) {
      numStr = "0" + numStr;
   }
   return numStr;
}
</pre>

<p>The following statements call the padZeros function.</p>

<pre class="brush: js">var result;
result = padZeros(42,4); // returns "0042"
result = padZeros(42,2); // returns "42"
result = padZeros(5,4);  // returns "0005"
</pre>

<h3 id="Determining_whether_a_function_exists">Determining whether a function exists</h3>

<p>You can determine whether a function exists by using the <code>typeof</code> operator. In the following example, a test is performed to determine if the <code>window</code> object has a property called <code>noFunc</code> that is a function. If so, it is used; otherwise some other action is taken.</p>

<pre class="brush: js"> if ('function' === typeof window.noFunc) {
   // use noFunc()
 } else {
   // do something else
 }
</pre>

<p>Note that in the <code>if</code> test, a reference to <code>noFunc</code> is used—there are no brackets "()" after the function name so the actual function is not called.</p>

<h2 id="Especificações">Especificações</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('ES1')}}</td>
   <td>{{Spec2('ES1')}}</td>
   <td>Initial definition. Implemented in JavaScript 1.0</td>
  </tr>
  <tr>
   <td>{{SpecName('ES5.1', '#sec-13', 'Function Definition')}}</td>
   <td>{{Spec2('ES5.1')}}</td>
   <td> </td>
  </tr>
  <tr>
   <td>{{SpecName('ES6', '#sec-function-definitions', 'Function definitions')}}</td>
   <td>{{Spec2('ES6')}}</td>
   <td>New: Arrow functions, Generator functions, default parameters, rest parameters.</td>
  </tr>
  <tr>
   <td>{{SpecName('ESDraft', '#sec-function-definitions', 'Function definitions')}}</td>
   <td>{{Spec2('ESDraft')}}</td>
   <td> </td>
  </tr>
 </tbody>
</table>

<h2 id="Compatibilidade_de_navegador">Compatibilidade de navegador</h2>



<p>{{Compat("javascript.functions")}}</p>

<h2 id="Consulte_também">Consulte também</h2>

<ul>
 <li>{{jsxref("Statements/function", "function statement")}}</li>
 <li>{{jsxref("Operators/function", "function expression")}}</li>
 <li>{{jsxref("Statements/function*", "function* statement")}}</li>
 <li>{{jsxref("Operators/function*", "function* expression")}}</li>
 <li>{{jsxref("Function")}}</li>
 <li>{{jsxref("GeneratorFunction")}}</li>
 <li>{{jsxref("Functions/Arrow_functions", "Arrow functions")}}</li>
 <li>{{jsxref("Functions/Default_parameters", "Default parameters")}}</li>
 <li>{{jsxref("Functions/rest_parameters", "Rest parameters")}}</li>
 <li>{{jsxref("Functions/arguments", "Arguments object")}}</li>
 <li>{{jsxref("Functions/get", "getter")}}</li>
 <li>{{jsxref("Functions/set", "setter")}}</li>
 <li>{{jsxref("Functions/Method_definitions", "Method definitions")}}</li>
 <li><a href="/en-US/docs/Web/JavaScript/Reference/Functions_and_function_scope">Functions and function scope</a></li>
</ul>