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The function declaration (function statement) defines a function with the specified parameters.

You can also define functions using the Function constructor and a function expression.

Syntax

function name([param[, param,[..., param]]]) {
   [statements]
}
name
The function name.
param
The name of an argument to be passed to the function. Maximum number of arguments varies in different engines.
statements
The statements which comprise the body of the function.

Description

A function created with a function declaration is a Function object and has all the properties, methods and behavior of Function objects. See Function for detailed information on functions.

A function can also be created using an expression (see function expression).

By default, functions return undefined. To return any other value, the function must have a return statement that specifies the value to return.

Conditionally created functions

Functions can be conditionally declared, that is, a function statement can be nested within an if statement, however the results are inconsistent across implementations and therefore this pattern should not be used in production code. For conditional function creation, use function expressions instead.

var hoisted = "foo" in this;
console.log(`'foo' name ${hoisted ? "is" : "is not"} hoisted. typeof foo is ${typeof foo}`);
if (false) {
  function foo(){ return 1; }
}

// In Chrome: 
// 'foo' name is hoisted. typeof foo is undefined
// 
// In Firefox:
// 'foo' name is hoisted. typeof foo is undefined
//
// In Edge:
// 'foo' name is not hoisted. typeof foo is undefined
// 
// In Safari:
// 'foo' name is hoisted. typeof foo is function

The results are exactly the same for a condition that evaluates to true

var hoisted = "foo" in this;
console.log(`'foo' name ${hoisted ? "is" : "is not"} hoisted. typeof foo is ${typeof foo}`);
if (true) {
  function foo(){ return 1; }
}

// In Chrome: 
// 'foo' name is hoisted. typeof foo is undefined
// 
// In Firefox:
// 'foo' name is hoisted. typeof foo is undefined
//
// In Edge:
// 'foo' name is not hoisted. typeof foo is undefined
// 
// In Safari:
// 'foo' name is hoisted. typeof foo is function

Function declaration hoisting

Function declarations in JavaScript are hoisted to the top of the enclosing function or global scope. You can use the function before you declared it:

hoisted(); // logs "foo"

function hoisted() {
  console.log('foo');
}

Note that function expressions are not hoisted:

notHoisted(); // TypeError: notHoisted is not a function

var notHoisted = function() {
   console.log('bar');
};

Examples

Using function

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

function calc_sales(units_a, units_b, units_c) {
   return units_a * 79 + units_b * 129 + units_c * 699;
}

Specifications

Specification Status Comment
ECMAScript Latest Draft (ECMA-262)
The definition of 'Function definitions' in that specification.
Draft  
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Function definitions' in that specification.
Standard  
ECMAScript 5.1 (ECMA-262)
The definition of 'Function definition' in that specification.
Standard  
ECMAScript 3rd Edition (ECMA-262)
The definition of 'Function definition' in that specification.
Standard  
ECMAScript 1st Edition (ECMA-262)
The definition of 'Function definition' in that specification.
Standard Initial definition. Implemented in JavaScript 1.0.

Browser compatibility

Update compatibility data on GitHub
DesktopMobileServer
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidEdge MobileFirefox for AndroidOpera for AndroidiOS SafariSamsung InternetNode.js
Basic supportChrome Full support YesEdge Full support YesFirefox Full support 1IE Full support YesOpera Full support YesSafari Full support YesWebView Android Full support YesChrome Android Full support YesEdge Mobile Full support YesFirefox Android Full support 4Opera Android Full support YesSafari iOS Full support YesSamsung Internet Android Full support Yesnodejs Full support Yes
Allowed in sloppy modeChrome Full support 49Edge ? Firefox ? IE ? Opera Full support YesSafari ? WebView Android ? Chrome Android ? Edge Mobile ? Firefox Android ? Opera Android ? Safari iOS ? Samsung Internet Android ? nodejs ?
Trailing comma in parametersChrome Full support 58Edge ? Firefox Full support 52IE ? Opera Full support 45Safari ? WebView Android Full support 58Chrome Android Full support 58Edge Mobile ? Firefox Android Full support 52Opera Android Full support 45Safari iOS ? Samsung Internet Android Full support 7.0nodejs Full support 8.0.0

Legend

Full support  
Full support
Compatibility unknown  
Compatibility unknown

See also

Etiquetas do documento e contribuidores

Última atualização por: fscholz,