Microsoft AJAX Library - 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 defining 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.

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 modified to reflect 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.

NOTE: NET languages such as C# and VB.NET support similar functionality through the concept of delegates. A delegate is a data type that represents a reference to a function. An instance of a delegate represents a function instance, and it can be passed as a parameter to methods that need to execute that function. Delegates are the technical means used by .NET to implement event-handling. C# 2.0 added support for anonymous methods, which behave similarly to JavaScript anonymous functions.

Functions as Variables

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