Common Mistakes, Debugging, and Error Handling - Beginning JavaScript - page 89

If you call this function, you get an error message similar to the one shown in Figure 4-2.
Figure 4-2
The error here is actually a simple typo in the function defi nition. The fi rst parameter has the typo: it
should read parameterOne , not parametrOne . What can be confusing with this type of error is that
although the browser tells us the error is on one line, the source of the error is on another line.
Case Sensitivity
This is a major source of errors, particularly because it can be diffi cult to spot at times.
For example, spot the three case errors in the following code:
var myName = “Jeremy”;
If (myName == “jeremy”)
alert(myName.toUppercase());
The fi rst error is the if keyword; the code above has If rather than if . However, JavaScript won't tell
us that the error is an incorrect use of case, but instead IE will tell us Object expected and Firefox will
tell us that If is not defined . Although error messages give us some idea of what's gone wrong, they
often do so in an oblique way. In this case IE thinks you are trying to use an object called an If object
and Firefox thinks you are trying to use an undefi ned function called If .
Okay, with that error cleared up, you come to the next error, not one of JavaScript syntax, but a logic
error. Remember that Jeremy does not equal jeremy in JavaScript, so myName == “jeremy” is false ,
even though it's quite likely that you didn't care whether the word is jeremy or jeremy . This type of
error will result in no error message at all, just the code not executing as you'd planned.
The third fault is with the toUpperCase() method of the String object contained in myName . The
previous code uses toUppercase , with the C in lowercase. IE will give us the message Object doesn't
support this property or method and Firefox will report that myName.toUppercase is not a
function . On fi rst glance it would be easy to miss such a small mistake and start checking your JavaScript
reference guide for that method. You might wonder why it's there, but your code is not working. Again,
you always need to be aware of case, something that even experts get wrong from time to time.