Functions - JavaScript by Example - page 171

EXAMPLE 7.13 ( CONTINUED )

if(err=="Error2"){
alert("Error! You didn't enter your age");

}
if(err=="Error3"){
alert("Error! The value is not a number");

}

}

7 if (age < 13){

alert("You pay child's fare!");

}
else if(age < 55 ){
alert("You pay adult fare.");

}
else { alert("You pay senior fare.");
}
</script>
</body>
</html>

EXPLANATION
1

This simple example is here to show you how the try/catch/throw statements can
be used for handling errors. For something like this, you will probably find it eas-
ier to test the program in the if conditionals, but we'll use it to demonstrate how
to throw exceptions. First, the user is prompted for his or her age.

2 The try block is entered to test for the possible error conditions defined by the

programmer.

3 If an invalid age is entered (i.e., an age less than 0 or greater than 120). An error

string, “Error1” is thrown and picked up in the catch block to be tested.

4 If the user didn't enter anything, the string “Error2” is thrown and picked up in

the catch block.

5 In the catch block, the errors will be handled based on the error string that was

thrown from the try block.

6 The err value being passed to the catch block is one of the values that was thrown

from the try block; for example, “Error1”, “Error2”, “Error3”.

7 Because all the testing and error handling was handled in the try/catch blocks, the

rest of the program can continue with no further testing of the age variable (see
Figure 7.22).