JavaScript Functions

1. Declaration
   1. Defined with function

      function rectangleArea(width, height) {
      	return width * height;
      }

   2. Return value is optional
   3. Can be defined before or after function call
2. Variable scope
   1. Local variables must be defined with var

      function sayHello() {
      	var name = "John";
      	alert("Hello, " + name);
      }
      
      sayHello();
      alert(name);   // ERROR! name is not defined 
      

   2. Missing var makes variable global

      function sayHello() {
      	name = "John";    // Global variable
      	alert("Hello, " + name);
      }
      
      sayHello();
      alert(name);   // John
      

   3. Global variables are actually attached to window object

      alert(window.name);   // John
      

   4. Best practice is to always declare every variable
3. Optional and variable-length arguments
   1. Test to see if the parameter has been defined

      function sayHello(name) {
      	name = (typeof name === "undefined") ? "John" : name;	
      	alert("Hello, " + name);
      }
      
      sayHello("Bob");  // Hello, Bob
      sayHello();       // Hello, John
      

   2. Coming in ECMAScript 6

      // Only works in FireFox
      function sayHello(name = "John") {	
      	alert("Hello, " + name);
      }
      

   3. arguments property gives array-like access to parameters

      function add() {
      	var result = 0;
      	for (var i = 0; i < arguments.length; i++)
      		result += arguments[i];
      		
      	return result;
      }
      
      x = add(2, -1, 3);  // 4
      
      

4. Anonymous functions
   1. Named functions may be assigned to a variable

      function sayHello() { alert('Hello'); }
      
      var x = sayHello;
      x();        // Hello

   2. Anonymous function can also be assigned

      var sayHelloAnonymous = function() { alert('Hello'); }
      sayHelloAnonymous();    // Hello
      
      var area = function(width, height) { return width * height; }
      x = area(2, 3);   // 6

   3. Anonymous function that executes immediately - Immediately Invoked Function Expression (IIFE)

      (function(name) { alert('Hello, ' + name); }('Bob'));
      
      // or
      (function(name) { alert('Hello, ' + name); })('Bob');
      

   4. Anonymous functions as function arguments

      // Register an event listener
      my button = document.getElementById('mybutton');
      button.addEventListener('click', function() { 
      	alert('Button was clicked'); 
      });
      
      // Loop through an array
      var nums = [2, 4, 6, 8];
      nums.forEach(function(element, index, array) {
      	console.log(index + ' = ' + element);
      });