Function expression

The function keyword can be used to define a function inside an expression.

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

The expression is not allowed at the start of a statement.

function [name]([param1[, param2[, ..., paramN]]]) {
   statements
}

As of ES2015, you can also use arrow functions.

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 an argument to be passed to the function.

statements Optional

The statements which comprise the body of the function.

A function expression is very similar to and has almost the same syntax as a function declaration (see 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 functions for more information.

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');
};

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 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 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)

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!')
})

An anonymous function is created and called:

(function() {
    console.log('Code runs!')
})();

• Arrow functions
• Functions and function scope
• Function
• function statement
• function* statement
• function* expression
• GeneratorFunction
• async function
• async function expression