Beginning Ajax With ASP.NET (2006).pdf

Chapter 2
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Examples of compiled programs are as simple as the Notepad application you’ll be using to edit
the samples shown in this chapter. Notepad was written in C++ and compiled to run specifically
on 32-bit or 64-bit Intel hardware. Therefore, it is very specific as to what platform can run
the code.
JavaScript on the other hand is interpreted, meaning that any platform capable of interpreting the
code can run it. For our purposes, this will mean virtually any web browser, from small handheld
devices like web-enabled phones up to browsers available for mainstream operating systems
like Internet Explorer, Opera, Safari, and Firefox. Some languages, such as .NET and Java, are
actually hybrids, being both partially compiled and partially interpreted.
Although it’s tempting to think that JavaScript is closely related to Java, because of its name, surprisingly
that is not the case. The only real similarities between the two are that the structure of the code
looks very similar, with the curly braces and semicolons, and they both provide an object-oriented programming
experience. Java does this more elegantly and has established itself as a great academic tool to
experiment with object-oriented code design. But the object model is very different from JavaScript.
General Rules of JavaScript
JavaScript code comprises a series of statements. And JavaScript statements are composed of either text
that leads up to a semicolon or text that opens up a code block inside curly braces. Here’s an example of
a couple of statements that first declare a variable called message and then present it on the screen in a
pop-up box:
var message=”Hello, world!”;
alert(message);
The computer determines the end of each of the statement simply by where it finds a semicolon. These two
statements could have been placed on the same line with a semicolon separating them. But it is much
clearer to put them on separate lines.
Most JavaScript interpreters will actually allow statements to exist without the semicolon appearing at
the end of the line. But this is not in keeping with the specifications of other similar languages like Java,
C++, and C#. So, it’s recommended to keep the habit of using semicolons at the end of statements. All of
the JavaScript examples in this book will do so, but when you delve into other samples from the Internet,
you will undoubtedly find JavaScript code that omits the semicolons.
Now take a look at an example of a statement that opens a code block:
if(grade>=90)
{
alert(“You got an A!”);
}
In this sample, the curly braces define a code block, and the single line of code found within runs only if
the grade variable is greater than or equal to 90. There are two statements shown, if and alert, and
there is no concluding semicolon required after the code block. In addition to having code blocks with
an if statement, you’ll see in upcoming samples that they are also found with the looping statement for
and when functions get defined.
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