How JavaScript functions work

This code resides in a fi le named JavaScriptDom.js, which is referenced from an HTML file (JavaScriptDom.html in the exercise), but it could have been included directly in a <script> tag of the HTML fi le. How it's stored is irrelevant; what does matter is that all that code is executed when it's read by the interpreter. If it was included in a function it would only execute if the function is called explicitly, as is this example:

// call function to display greeting message
ShowHelloWorld();

// "Hello, World" function

function ShowHelloWorld()
{

// declaring new variables
var date = new Date();
var hour = date.getHours();

// simple conditional content output
if (hour >= 22 || hour <= 5)
      document.write("Goodnight, world!");
else
      document.write("Hello, world!");

}

This code has the same output as the previous version of the code, but it is only because the ShowHelloWorld() function is called that will display "Goodnight, world!" or "Hello, world!" depending on the hour of the day. Without that function call, the JavaScript interpreter would take note of the existence of a function named ShowHelloWorld(), but would not execute it.

Functions as Variables

In JavaScript, functions are first-class objects. This means that a function is regarded as a data type whose values can be saved in local variables, passed as parameters, and so on. For example, when defi ning a function, you can assign it to a variable, and then call the function through this variable. Take this example:

// displays greeting
var display = function DisplayGreeting(hour)
{
     if (hour >= 22 || hour <= 5)
            document.write("Goodnight, world!");
     else
            document.write("Hello, world!");
}

// call DisplayGreeting supplying an hour as parameter
display(10);

When storing a piece of code as a variable, as in this example, it can make sense to create it as an anonymous function—which is, a function without a name. You do this by simply omitting to specify a function name when creating it:

// displays greeting
var display = function(hour)
{
...
}

Anonymous functions will come in handy in many circumstances when you need to pass an executable piece of code that you don't intend to reuse anywhere else, as parameter to a function.

Let's see how we can send functions as parameters. Instead of sending a numeric hour to DisplayGreeting(), we can send a function that in turn returns the current hour. To demonstrate this, we create a function named GetCurrentHour(), and send it as parameter to DisplayGreeting(). DisplayGreeting() needs to be modifi ed to refl ect that its new parameter is a function—it should be referenced by appending parentheses to its name. Here's how:

// returns the current hour
function GetCurrentHour()
{
   // obtaining the current hour
   var date = new Date();
   var hour = date.getHours();
   // return the hour
    return hour;
}

// display greeting
function DisplayGreeting(hourFunc)
{
    // retrieve the hour using the function received as parameter
    hour = hourFunc();
    // display greeting
    if (hour >= 22 || hour <= 5)
          document.write("Goodnight, world!");
    else
          document.write("Hello, world!");
}

// call DisplayGreeting
DisplayGreeting(GetCurrentHour);

This code can be tested online at http://www.cristiandarie.ro/asp-ajax/Delegate.html. The output should resemble Figure 3-1.

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