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The Power of Javascript: Controlling the Execution of the Script


When you write a script, most of the time you need to make a decision based on the values of some of your variables, or based on the values entered by the users, or even based on user actions. Decision making is a fundamental building block for all programming languages and it's similar in most of them. In this article, we discuss the if/else statement block, the switch statement, and take a look at the prompt() dialog box.

Author Info:
By: Michael Youssef
Rating: 5 stars5 stars5 stars5 stars5 stars / 12
August 09, 2005
TABLE OF CONTENTS:
  1. · The Power of Javascript: Controlling the Execution of the Script
  2. · The if statement
  3. · The if statement example
  4. · Advanced if statements (else if and else)
  5. · The switch statement
  6. · The switch example

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The Power of Javascript: Controlling the Execution of the Script - The switch example
(Page 6 of 6 )

Copy (or write) the following code into an HTML document and load it.

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
  <head>
    <title>Hello World</title> 
    <script language="JavaScript" type="text/javascript">
      var someText;
      someText = prompt("Please enter some text", "I like Javascript");
      switch(someText)
      {
        case "I like Javascript":
          alert("You have written, " + someText);
          break;

        case "I like scritping languages":
          alert("You have written, " + someText);
          break;

        case "I like Perl":
          alert("You have written, " + someText);
          break;

        default:
          alert("You have written, " + someText);
          break;
      }
      document.write("<h3>This is the first statement after the switch statement</h3>");
    </script>
  </head>
  <body>
  </body>
</html>

Enter the value "I like Perl" into the prompt box.

A message box is displayed telling you what you have written.

And the following line is written to the Web page.

In our script we have a switch statement which tests the variable someText for the possible values that we enter. When you entered the string value "I like Perl" the interpreter assigned this value to the variable someText, then in the switch statement it goes through the case sections one by one until a match is found in the third case section. The Interpreter then executes this case's statements which display the message box. Let's do something else with this example. Remove the default section from the switch statement so it will look like this:

switch(someText)
{
  case "I like Javascript":
    alert("You have written, " + someText);
    break;

  case "I like scritping languages":
    alert("You have written, " + someText);
    break;

  case "I like Perl":
    alert("You have written, " + someText);
    break;
}

Now save and reload the document and type the string value "hi" into the prompt's text box, then click OK. As you can see there's no message box telling you about the value you have written because we removed the default section. The interpreter goes through the case sections, but does not find any match, so it terminates the switch block and starts at the following statement in the script.     


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