functions.md

title	level	source	attribution
Functions	beginner	http://jqfundamentals.com/legacy	jQuery Fundamentals

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"