Despite our best intentions and wishes, programs sometimes fail. In Chapter 5, "Validating Your Program," you learned about ways the compiler can catch your typos and invalid requests: syntax errors and type mismatches. These are kinds of compile-time errors. But there are countless kinds of errors that can happen while your program is running, and these may be the result of a logical error in your code or just the result of unexpected circumstances out of your control. Whichever is the case, ActionScript 3.0 gives you the opportunity to handle these errors at runtime. This chapter provides the tools for you to understand runtime errors and keep them from affecting the end user.
If a certain function fails, there are a few ways for it to signal this failure. Some functions return a value that indicates that the function did not complete successfully. Code that uses this function must then check to make sure all went as planned before continuing. This approach has a few problems, though.
First, returning a Boolean signifying whether the function call was successful is not always possible. What if the function is already supposed to return a value? If you are returning a complex object, setting the value of this returned object to
null is one option. If you are returning an
int, you must ...