--- title: Expressão função (Function expression) slug: Web/JavaScript/Reference/Operators/function tags: - Expressões Primárias - Funiconaldiade de Linguagem - Função - JavaScript - Operador translation_of: Web/JavaScript/Reference/Operators/function original_slug: Web/JavaScript/Reference/Operadores/função ---
{{jsSidebar("Operators")}}

A palavra-chave function  pode ser utilziada para definir uma função dentro de uma expressão.

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

{{EmbedInteractiveExample("pages/js/expressions-functionexpression.html", "shorter")}}

Sintaxe

let myFunction = function [name]([param1[, param2[, ..., paramN]]]) {
   statements
};

As of ES2015, you can also use {{jsxref("Functions/Arrow_functions", "arrow functions")}}.

Parâmetros

name {{optional_inline}}
The function name. Can be omitted, in which case the function is anonymous. The name is only local to the function body.
paramN {{optional_inline}}
The name of an argument to be passed to the function.
statements {{optional_inline}}
The statements which comprise the body of the function.

Descrição

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 function name, which can be omitted in function expressions to create anonymous functions. A function expression can be used as an IIFE (Immediately Invoked Function Expression) which runs as soon as it is defined. See also the chapter about {{jsxref("Functions", "functions")}} for more information.

Function expression hoisting

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:

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');
};

Named function expression

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 {{jsxref("Functions/arguments/callee", "arguments.callee")}} property.

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;

The variable the function expression is assigned to will have a name 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).

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)

Exemplos

The following example defines an unnamed function and assigns it to x. The function returns the square of its argument:

var x = function(y) {
   return y * y;
};

More commonly it is used as a callback:

button.addEventListener('click', function(event) {
    console.log('button is clicked!')
})

Especificações

Especificação
{{SpecName('ESDraft', '#sec-function-definitions', 'Function definitions')}}

Compatibilidade de navegador

{{Compat("javascript.operators.function")}}

Consulte também