Exception handling in Javascript.

Handling execptions statements

Using the try, catch and finally statements

• The try, catch and finally statement is JavaScript's exception-handling mechanism.
• A try, catch and finally statement has the form:

   try {
      statements
    } catch (variable) {
      statements
    } finally {
      statements
    }

  Where :
  • The try clause of this statement simply defines the block of code whose exceptions are to be handled.
  • The catch clause is the block of statements that are invoked when an exception occurs anywhere within the try block.
  • The catch clause is followed by a finally block, which normally contains cleanup codes. The finally blocks are optional, but is guaranteed to be executed either an exception or not.
  An example:

  <script type="text/javascript">
    var age = 0;
    try {
      // Execute a function that does not exist;
      age= getAge("Let me know your age: ");
      // This statement will never be executed:
       document.write("Your age is " + age);
    }catch (ex) {  // fetch the error (ex is the error message)
      // Tell the user what the error is
      document.write("<h3 style='color: red;'>"+ex+"</h3>");
    }finally {
      document.write("Maybe I should not asked you about your age.");
    }
  </script>

Using the throw statement.

To throw an exception is to signal that an error or an exceptional condition.
• A throw statement has the form:

  throw expression;throw expression;

• Exceptions are thrown by the system whenever a runtime error occurs and whenever the program explicitly throws one using the throw statement.
  An example:

  <script type="text/javascript">
    var n = 0;
    var approved=false;
    while (!approved) {
      try {
        // Ask the user to enter a number
        n = prompt("To enter an integer between 0 and 10 makes no exceptions.", 0);
        // Compute the factorial of the number, assuming that the user's
        // input is valid
        if (n<=0 || n>=10) {
          throw "Your integer has an error value: "+n;
        }
        // Display the result
        document.write("OK n = " + n+"<br>");
        approved=true;
      }
      catch (ex) {  // If the user's input was not valid, we end up here
        // Tell the user what the error is
        alert(ex);
      }finally {
        if (confirm("Try again?")) { approved=false;
        }else {  approved=true; }
      }
    }
  </script>