Functions

Functions contain blocks of code that need to be executed repeatedly. Functions can take zero or more arguments, and can optionally return a value.

Functions can be created in a variety of ways, two of which are shown below:

                
                                // Function Declaration
                              
                                function foo() {
                              
                                /* do something */
                            
                              }

                
                                // Named Function Expression
                              
                              var foo = function() {
                            
                                /* do something */
                            
                              }

Using Functions

                
                                // A simple function
                              
                              var greet = function( person, greeting ) {
                            
                                var text = greeting + ", " + person;
                            
                                console.log( text );
                            
                              };
                            
                              greet( "Rebecca", "Hello" );

                
                                // A function that returns a value
                              
                              var greet = function( person, greeting ) {
                            
                                var text = greeting + ", " + person;
                            
                                return text;
                            
                              };
                            
                              console.log( greet( "Rebecca", "hello" ) ); // "hello, Rebecca"

                
                                // A function that returns another function
                              
                              var greet = function( person, greeting ) {
                            
                                var text = greeting + ", " + person;
                            
                                return function() {
                            
                                  console.log( text );
                            
                                };
                            
                              };
                            
                              var greeting = greet( "Rebecca", "Hello" );
                            
                              greeting();

Immediately-Invoked Function Expression (IIFE)

A common pattern in JavaScript is the immediately-invoked function expression. This pattern creates a function expression and then immediately executes the function. This pattern is extremely useful for cases where you want to avoid polluting the global namespace with code — no variables declared inside of the function are visible outside of it.

                
                                // An immediately-invoked function expression
                              
                              (function() {
                            
                                var foo = "Hello world";
                            
                              })();
                            
                              console.log( foo );   // undefined!

Functions as Arguments

In JavaScript, functions are "first-class citizens" — they can be assigned to variables or passed to other functions as arguments. Passing functions as arguments is an extremely common idiom in jQuery.

                
                                // Passing an anonymous function as an argument
                              
                              var myFn = function( fn ) {
                            
                                var result = fn();
                            
                                console.log( result );
                            
                              };
                            
                                // logs "hello world"
                              
                              myFn( function() {
                            
                                return "hello world";
                            
                              });

                
                                // Passing a named function as an argument
                              
                              var myFn = function( fn ) {
                            
                                var result = fn();
                            
                                console.log( result );
                            
                              };
                            
                              var myOtherFn = function() {
                            
                                return "hello world";
                            
                              };
                            
                              myFn( myOtherFn ); // "hello world"

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