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Basic JavaScript with Examples
A tutorial with reusable code snippets
By Reaz Hoque
// originally appeared in IDM v2n13 (June 1997)

Since the day Microsoft built support for JavaScript into Internet Explorer 3.0, Netscape's client-side language has become the de facto standard for enhancing web pages at the browser.

In this full-length excerpt from Practical JavaScript Programming, author Reaz Hoque explains the basics of client-side scripting. He also gives you some neat scripts that can enhance your web pages, making them impressively interactive. Important code snippets available in this article include a browser detection script, capable of offering tailored content based on broswer version or type. For instance, it's important (unless you like crashing visitors' computers) to only serve JavaScript 1.1 code to Netscape Navigator 3.0 or later.

By simply cutting and pasting the code examples below, you'll be well on your way to making your web site a more valuable, and more enjoyable, place to visit.

Your first JavaScript script

In any language, the programmer needs to find out how to output text into a document. In order to output text in JavaScript you must use write() or writeln(). Here's an example:

Writing text on a document 

<HTML>
<HEAD>
<TITLE>Writing text on a document</TITLE>
</HEAD>
<BODY>
<SCRIPT LANGUAGE="JAVASCRIPT">
<!-- Hiding the code
document.write("Welcome to Applications in JavaScript!");
// done hiding -->
</SCRIPT>
</BODY>
</HTML>

Test This Example

If you're familiar with JavaScript syntax, this script should be easy for you to understand. You will notice that we have included the code in between <SCRIPT> and </SCRIPT> tags. We made sure the document object write is in lowercase as JavaScript is case sensitive.

Now, you might be wondering what the difference is between write and writeln. Well, while write just outputs a text, writeln outputs a line feed after the text output.

Date and time

You probably have seen many Web sites that incorporate a clock. Before JavaScript, you had to use CGI to have the current date and time on a page. Now, creating clocks and the date is easy and can make your site look cool. Using this example, you also can create custom greetings for your visitor, depending on the time of day. For example, you can have a message saying, "It's 12:00 a.m.. Go to bed!"

If you look at the Netscape's JavaScript documentation, you will know that there are two built-in functions in JavaScript that let you display date and time. The following example will display the current time and date in a text box with an option to stop the time.

  

<HTML>
<HEAD>
<TITLE>Showing date and time on a document</TITLE>
<SCRIPT LANGUAGE="JAVASCRIPT">
<!--Hiding the code
var show_time=false;
var timerID=null;

function stop(){
   if (show_time){
       clearTimeout(timerID);
       document.clock.date_time.value=" ";
   }
   show_time=false;
}

function start(form){
   var today=new Date();
   var display_value =" Time= " + today.getHours()
   if(today.getMinutes() < 10){
      display_value+=":0" + today.getMinutes();
   }
   else{
      display_value+=":" + today.getMinutes();
   }
   if (today.getSeconds() < 10){
      display_value+=":0" + today.getSeconds();
   }
   else{
      display_value+=":" + today.getSeconds();
   }
   if(today.getHours()>=12){
      display_value+=" P.M."
      /* Here we have a variable called mygreeting where
         you can store somthing like this:
         mygreeting ="Good Morning!"; */
   }
   else{
      display_value+=" A.M."
      /* Now set mygreeting to:
         mygreeting ="Good Afternoon!"; */
   }
   display_value += " Date= " + (today.getMonth()+1) + "/"  +
      today.getDate() + "/" + today.getYear();
      document.clock.date_time.value=display_value;
      timerID=setTimeout("start()",100);
      show_time=true;
      /* Here have an alert() method do the following:
         alert(mygreeting); */
}
//done hiding-->
</SCRIPT>
</HEAD>
<BODY BGCOLOR=white Text=Red Link=Green onLoad=stop()>
<center>
<H2>Displaying Date and Time</H2>
<FORM name=clock>
<INPUT type="text" name="date_time" size=35 value=" "><br>
<INPUT type="button" name="show_now" value="Display" onClick=start()>
<INPUT type="button" name="clear_now" value=" Clear " onClick=stop()>
</center>
</FORM>
</BODY>
</HTML>

Test This Example

In this script, we have two functions (stop() and start()) for our two buttons Display and Clear. When the button Display is pressed, the start() function is called, and when the button Clear is pressed, the stop() function is called. Both of these functions are called using the onClick event handler.

Now the stop() function simply clears the setTimeout method that is used in the start() function. This function also clears the text box that displays the time and date. Finally, the function puts a false value to the Boolean variable show_time.

In start() function, a new instance of date using the statement new is created. Then a variable display_value was defined so that we can export the date and time value in the text box later. Remember that we are using the display_value as a string variable. Now we have to make sure that if the seconds and minutes are less than 10, they are both displayed as single digits. The if...else statement is used for that purpose.

In order to display a.m. or p.m., another if...else statement is used to check for a value more than or equal to 12. So, if the time value is more than 12, display_value would then hold "p.m.," otherwise it would hold "a.m." Lastly, the value of the day, month, and the year is added in the variable. Now it's time to display the data into the text box. This is done with the following code: 

 document.clock.date_time.value=display_value;
The above code takes the value of display_value and puts it in the property called value of the text box named date_time. The word document is used to insure that we are indicating the current document. Lastly, the variable timerID is set and show_time is set to true. Notice that timeID uses the setTimeOut() function with the parameters start() and 100. The reason why 100 is passed in is because the function start() is within one tenth of the second.
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