Anonymous Functions

Anonymous functions in JavaScript are functions created without a name.

In JavaScript and its many frameworks, they are used prominently as callbacks.

javascript anonymous functions

A normal function declaration in JavaScript goes with the function statement first, followed by a user-given name, followed by a pair of opening/closing parentheses (with comma-separated parameters) and finally the body within a pair of braces

				
				function name() {
					console.log('This is a normal function.');
				}

But unlike normal function declarations, the user-defined name is omitted after the function statement for anonymous functions and followed by a pair of parentheses (with comma-separated parameters inside). An anonymous function can be assigned to a variable as

				
				var anonymous = function() {
					console.log('This is an anonymous function.');
				}

and can be invoked by attaching a pair of opening and closing parentheses (with comma-separated arguments inside) to the assigned variable as

				
				anonymous();

But unlike normal functions, you cannot call them before the declaration.

Anonymous Functions as Callbacks

Anonymous functions can also be passed as arguments to another function. They are then known as callback functions. Below, an anonymous function is passed as an argument to the JavaScript pre-defined function setTimeout() that prints a message after 1 second

					
					setTimeout(function() {
					  console.log('This message is printed after 1 second.');
					}, 1000);

Anonymous functions are also heavily used in jQuery. For example, the .ready() method is widely used by passing an anonymous function as

					
					$(document).ready(function(){
						// inside an anonymous function
					});

Anonymous Functions in Self-Invoking Functions

Inside the first pair of parentheses of a self-invoking function is an anonymous function. The succeeding pair of parentheses is the self-executing part which causes the anonymous function inside the preceding pair to be executed immediately

					
					(function(){
						console.log('This is a self-executing anonymous function.');
					})();