The JavaScript Component

AJAX technologies take advantage of the common support for JavaScript found in modern browsers. Because there is a standard that is supported across the various browsers, you can write scripts knowing that they will run. This wasn’t always the case.

In the mid 1990’s, Netscape and Microsoft (along with others) collaborated on a standard for a scripting language that they would support in their web browsers. The standard is called EcmaScript. Microsoft’s implementation is called JScript, but the language is generally referred to as JavaScript, as it was called in Netscape. (It has nothing to do with Java, but someone must have thought the association was useful for marketing purposes.) JavaScript program snippets are sent down to the browser along with the HTML, and they run inside the user’s browser to affect how the page is processed on the client.

JavaScript is not compiled; it is interpreted. There is no static type-checking like you get in C++ and C#. You can declare a variable without needing to specify a type, and the type to which the variable refers can change at any time. This makes it easy to get started programming in JavaScript, but there’s inevitably a certain amount of danger in allowing the data type of a variable to change dynamically at runtime. In the following snippet, notice that the variable can reference any type without difficulty:

var something = 1;
something = true;
something = “a string”;

JavaScript is a dynamic language. Types can actually be extended during program execution by other code. This means that you can write code that creates types on the fly. Because there is no enforcement of type safety, your code can receive these types as parameters or return values without any problem. This provides a great degree of flexibility and coding power.

The fundamental types in JavaScript are strings, numbers, Booleans, and functions. There is also support for objects and arrays, which are collections of the fundamental types. Some additional objects are included that are considered essential for many programming tasks. This includes support for regular expressions and date and time operations.

You can use the plus operator on strings in JavaScript to concatenate them:

var theEnd = “THE END.”;
var result = “Beginning, “ + “middle, and “ + theEnd;

In this example, the result variable is now the string: “Beginning, middle, and THE END.”

JavaScript interpreters use the IEEE floating-point standard for storing numbers. Ignoring the gory details, you can assume that for most programming tasks you won’t have any trouble.

The Boolean type in JavaScript is about what you would expect it to be but maybe not exactly so. The Boolean represents whether or not an expression is true, but it uses the C-style convention using integer values 0 and 1.

Variables can exist in JavaScript without having a value, and a variable may simply be undefined, which can produce unexpected results. In this piece of JavaScript, three variables are declared, and all of these comparisons are designed to return a true value.

You can check specifically to see if a variable has been defined like this:

if( typeof(undefinedVar ) == “undefined” )
{
       alert(“undefinedVar is undefined”);
}

Variables can also have a null value, which is not the same thing as being undefined, as a null value does constitute a value.

Functions are also real types in JavaScript. They can accept arguments and return values. Functions can be passed to other functions and can be created dynamically by other script code.

Here are two equivalent definitions for a function called Add that will take two variables and return the result of applying the plus operator. Notice that I didn’t state that it takes two numbers. Remember, JavaScript variables don’t have a defined type, so I could just as easily pass two strings and get them concatenated by my Add function.

Once either of these styles is used to create a function, it can be called from that scope and any nested scope to perform the addition. There is no advantage to one of these forms over the other. You can simply choose to use the syntax that you prefer.

Objects and arrays are just collections of other types. Array types do not require that the values they hold be named; instead, you can access them by index. The values held in an object are referenced by field or property names. Objects can also hold functions (which can be accessor functions to give public visibility to local variables), which lets you create data structures that represent entities in JavaScript code. Missing from this sort of object-oriented programming is a concept of type inheritance. The Microsoft AJAX Library provides a set of classes and recommended patterns for achieving inheritance in JavaScript, making it more natural for switching between JavaScript and other high-level languages. The following code example includes a definition for an Album object that holds and returns the artist and album title. An array is then used to store information about several albums.