JavaScript is fun.
Here is what this part of the tutorial covers:
Orientation
How the browser knows it is JavaScript. This is how JavaScript works. Some programming basics.
~~ Orientation
Let's clear up any confusion there may be between JavaScript and Java:
Although they have similar names, they are very different. Java is a compiled language with modules that load into your browser from a source external to the web page. JavaScript, on the other hand, is an interpreted language and is almost always an integral part of the web page source code.
"Compiled language" means the source code of the program is translated (compiled) into machine-language (composed only of 0's and 1's) before use. When it is time to run the program, the translated version is used by the computer instead of the original source code. Unless you are the programmer, you might never see the source code of the program you are using.
"Interpreted language" means when it loads a page, the browser's interpreter uses the original source code and translates it into machine language. The translation is stored in the browser's memory ready to run all or portions of it as appropriate. When the browser window gets ready to load a different web page, the previous page's translated JavaScript is discarded.
JavaScript can be easier to learn than some other languages ecause there is a great abundance of examples on the internet. Just use your browser's "view source" menu choice and you can see the source code of web pages.
Note: Some browsers will display only the active source code when you "view source". In other words, you see only the source code that is actually being used to display the page at the time you are viewing it.
If you suspect your browser is "censoring" parts of the source code on a particular page, you can use Master Snooper. Master Snooper shows you all of the code in its glory or lack thereof. Just go to http://willmaster.com/master/snooper/MasterSnooper.cgi and type in the URL of the web page. (Master Snooper is also good to see how framed, doorway, and redirection pages are built, because it shows you only the source code rather than drawing the display or getting itself redirected.)
JavaScript can be quick to develop because you can make a change and test it immediately. To set it up, load the source code file into both your word processor and your browser. When you can change something, save the file and click your browser's "reload/refresh" button for immediate feedback.
Be sure to use a plain text word processor. NotePad.exe and BBEdit are both good. I use Arachnophilia (CareWare [no monetary requirements] from http://www.arachnoid.com/arachnophilia -- Windows) for almost all of my HTML and JavaScript development.
~~ How the browser knows it is JavaScript:
JavaScript program lines must be between <script...> </script> tags unless the line is within an HTML tag. The JavaScript between the <script...> </script> tags must also be enclosed with HTML comment tags.
Often, you'll find JavaScript in this format:
<script
language="JavaScript">
<!-- programmer comments can go here
JavaScript program code goes here
// programmer comments can go here -->
</script>
The language="JavaScript" part is not specifically required. However it is good to get in the habit of typing it in because there are other script languages out there, Active X for example, and the language designation tells the browser which scripting language is being used.
Without the <!-- and --> HTML comment begin/end tags, the browser tries to display the JavaScript as regular text -- this last statement may not be true for the very latest browsers, but it is true for most recent browsers and it certainly is true for browsers that are not JavaScript enabled.
The second line contains the HTML comment begin tag. The JavaScript interpreter ignores the entire line so long as it begins with that tag. Some people put a copyright notice or other comments on the rest of that line.
Notice the characters
//
on the second from last line. Two slashes next to each other tell the browser's JavaScript interpreter that the rest of the line is not valid JavaScript code -- it is ignored the same as all programmer comments are ignored by the interpreter. The rest of the line contains, of course, the HTML end comment tag. (You can use the space between // and --> for comments of your own -- the JavaScript interpreter will ignore it, and it is still inside the HTML comment tag so the HTML interpreter will ignore it, too.)
You can put your comments anywhere in a JavaScript program so long as they begin with the characters: //
There is no "end" comment code for JavaScript. All JavaScript comments that begin with // end automatically at the end of that same line.
Sometimes, you'll find JavaScript within HTML tags:
For mouseovers and some other functionality, you will find JavaScript code within the anchor tag. Example:
<a
href="somedomain.com"
onMouseover="do_some_function()"
onMouseout="do_different_function()">
<img ...>
</a>
JavaScript code is also found within other HTML tags. Two of the most used are the <body...> and <input...> tags.
An example of JavaScript code in a <body...> tag:
<body
onLoad="do_some_function()">
An
example of JavaScript code in an <input...> tag:
<input
type="submit" onClick="do_some_function()">
The reason non-JavaScript enabled browsers don't get confused when they find JavaScript in HTML tags is because they are programmed to ignore anything in tags that they do not recognize.
~~ This is how JavaScript works:
When
someone visits a web page, it loads into the browser. The HTML is
converted into a visible display.
If the browser is JavaScript enabled then the JavaScript program lines are interpreted and executed. If the JavaScript directs any visible effects, those effects become visible in the page display.
Although all of thee JavaScript code is interpreted during loading not all of it will be executed right away.
JavaScript program lines within HTML tags are executed only when the link or form element is activated, such as the mouseover link example above. And functions are executed only when a program lines calls them.
~~ Some programming basics:
Where
to put JavaScript program code:
The JavaScript program lines above the <body...> tag are interpreted and appropriate lines executed before the HTML page is loaded. The JavaScript below the <body...> tag is interpreted and appropriate lines executed at the same time as the HTML page.
JavaScript program code that must be put above the <body...> tag are
(1) any that must execute before the entire page has finished loading, such as code that immediately determines what kind of browser the visitor is using, and
(2) functions (we'll describe what they are and other stuff about them in a later part of this series) that are called within the <body...> tag.
JavaScript program code that must be put below the <body...> tag are any that adjust the page's visual elements, such as printing text or displaying graphics.
Oftentimes, it won't matter whether the program code is above or below the <body...> tag. Many programmers, this author included, usually opt for the "above" position. It helps keep all or most of the program code in one place rather than scattered about.
How to make your program remember things:
You can specify stuff that the JavaScript program must remember. And you can access that remembered stuff later on.
In order to store stuff in memory and access it later, that memory spot must have a name. The memory spot, itself, is called a "variable" because the contents of the memory spot can change.
To declare that a variable exists and give it a name, you type something like
var blahblah;
which creates a variable called "blahblah" where you can store stuff.
To store something into that memory spot, you type something like
blahblah = 5;
and the number "5" is stored in (assigned to) that memory spot.
You can also do all the above in one line, if you want, by typing var blahblah = 5;
However, once a variable name has been declared, don't declare it again. If, later on in the program, you want to change the contents of the variable "blahblah", do it as a simple assignment statement, like
blahblah = 4;
To access what you have stored in a variable (the value you assigned to it), you either have to print it (have it show up on your web page) or assign it to another variable. To print it to your web page, you type
document.write(blahblah);
and, when interpreted and executed, that program line will print the value of "blahblah" on your web page.
Here it is, all put together:
<script
language="JavaScript">
<!--
var blahblah = 5;
document.write(blahblah);
// -->
</script>
Put
the above JavaScript code somewhere below the <body...> tag of a web page and you'll see how it prints the value of "blahblah". (You put it below the <body...> tag because it actually writes something to the page -- and some browsers protest if you try to write something to the page above the <body...> tag.)
The above demonstrates a simple use of a variable. Other uses (described in a later part of this series) include doing mathematical calculations, manipulating strings of characters, storing form field contents, and having your program make decisions based on what a variable contains.
Using strings of characters:
Strings of characters need to be enclosed between either apostrophes (') or quote marks (").
Whichever character you choose, it must be used both at the beginning and at the end of the string.
If you have one or more apostrophes within your string, it makes good sense to choose quote marks to contain the string. Example:
"I'm hot."
On the other hand, if you have one or more quote marks within your string, it makes sense to enclose it within apostrophes, like this:
'He said, "hot".'
A special situation arises when you have a string with both an apostrophe and a quote mark within it. In that case, you put a backslash character in front of each occurrence of the character that encloses the string. For example, the string of characters
He said, "I'm hot."
can be enclosed within apostrophes as
'He said, "I\'m hot."'
or within quote marks as
"He said, \"I'm hot.\""
The backslash tells the browser's interpreter to treat the character following the backslash as a literal character rather than an "end of string" marker. Once the interpreter has done that, it discards the backslash from the string.
(If you want to use a backslash in a string, you must use two of them in sequence -- the browser will discard the first one.) Try this JavaScript program code in a web page to see it work:
<script language="JavaScript">
<!--
var hotstuff = 'He said, "I\'m hot."';
document.write(hotstuff);
// -->
</script>
~~ To come:
This article has reached its length limit.
The next article in this series will describe useful things to do with variables and will also describe functions and how to call them.
Happy Happy!
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William Bontrager, Programmer and Publisher
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