function expression
Baseline Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
The function
keyword can be used to define a function inside an expression.
You can also define functions using the function
declaration or the arrow syntax.
Try it
Syntax
function (param0) {
statements
}
function (param0, param1) {
statements
}
function (param0, param1, /* …, */ paramN) {
statements
}
function name(param0) {
statements
}
function name(param0, param1) {
statements
}
function name(param0, param1, /* …, */ paramN) {
statements
}
Note: An expression statement cannot begin with the keyword function
to avoid ambiguity with a function
declaration. The function
keyword only begins an expression when it appears in a context that cannot accept statements.
Parameters
name
Optional-
The function name. Can be omitted, in which case the function is anonymous. The name is only local to the function body.
paramN
Optional-
The name of a formal parameter for the function. For the parameters' syntax, see the Functions reference.
statements
Optional-
The statements which comprise the body of the function.
Description
A function
expression is very similar to, and has almost the same syntax as, a function
declaration. 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 functions for more information.
Function expression hoisting
Function expressions in JavaScript are not hoisted, unlike function declarations. You can't use function expressions before you create 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 avoids using the deprecated arguments.callee
property to call the function recursively.
const math = {
factit: function factorial(n) {
console.log(n);
if (n <= 1) {
return 1;
}
return n * factorial(n - 1);
},
};
math.factit(3); //3;2;1;
If a function expression is named, the name
property of the function is set to that name, instead of the implicit name inferred from syntax (such as the variable the function is assigned to).
Unlike declarations, the name of the function expressions is read-only.
function foo() {
foo = 1;
}
foo();
console.log(foo); // 1
(function foo() {
foo = 1; // TypeError: Assignment to constant variable.
})();
Examples
Using function expression
The following example defines an unnamed function and assigns it to x
. The function returns the square of its argument:
const x = function (y) {
return y * y;
};
Using a function as a callback
More commonly it is used as a callback:
button.addEventListener("click", function (event) {
console.log("button is clicked!");
});
Using an Immediately Invoked Function Expression (IIFE)
An anonymous function is created and called:
(function () {
console.log("Code runs!");
})();
// or
!function () {
console.log("Code runs!");
}();
Specifications
Specification |
---|
ECMAScript Language Specification # sec-function-definitions |
Browser compatibility
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