You should also include enough information in the error message for the caller to construct their own error message without having to parse yours. They may want to localize the error message, or aggregate a large number of errors together, or display the error message differently e. Sometimes funcA will retry instead. In this case, consider wrapping the Error instead of returning it directly. By wrapping, we mean propagating a new Error that includes all of the information from the lower level, plus additional helpful context based on the current level.
The verror module provides an easy way to do this. Maybe you call this function when your server starts up. The whole path at startup looks like this:. If the connection step at 1. On the other hand, if each level wraps the Error returned from the lower level, you can get a much more informative message:. This example is conceptually simple, but demonstrates a bunch of the suggestions we talked about:.
All advice should be shrink-to-fit, and you should use your judgment if something truly is simple, but remember: ten minutes documenting expectations now may save hours for you or someone else later.
People sometimes write code like this when they want to handle the asynchronous error by invoking the callback function and passing the error as an argument. But they make the mistake of thinking that if they throw it from their own callback the function passed to doSomeAsynchronousOperation , then it can be caught in the catch block. That means the try block has been exited. The callback is invoked directly by Node, with no try block around it.
In JavaScript, you can technically throw things that are not Errors, but this should be avoided. The concepts of an operational error and a programmer error long predate Node.
In Java, this loosely tracks the use of checked and unchecked exceptions, though operational errors that are known to be unhandleable, like OutOfMemoryError , are grouped with unchecked exceptions. And it was terrible. Want to learn more about our cloud offerings? Request a Cloud Specialist.
It appears you have an ad or script blocker that won't allow us to load our feedback form from app-sjf. To submit feedback, you can either temporarily unblock that domain, or email marketing joyent. Email may take longer to get to the relevant people. Production Practices. Triton Node. Share: Error Handling in Node. This document will answer several questions that programmers new to Node. What should my functions assume about their arguments? Should I check more specific constraints, like that an argument is non-null, is non-negative, looks like an IP address, or the like?
Should I throw an exception or emit an error to the callback? How can I programmatically distinguish between different kinds of errors e. How can I provide enough detail with my errors so that callers can know what to do about them? How should I handle unexpected errors? Operational errors vs. These are not bugs in the program. In fact, these are usually problems with something else: the system itself e. Examples include: failed to connect to server failed to resolve hostname invalid user input request timeout server returned a response socket hang-up system is out of memory Programmer errors are bugs in the program.
These are things that can always be avoided by changing the code. They can never be handled properly since by definition the code in question is broken. This way, you'll have more control over your program's flow:. I usually use the first way for simple debugging in development code. It is not recommended for production. The best way is to throw an exception, which you can catch in other parts of the program and do some error handling on.
The three styles are not drop-in replacements for each other. The first one is not an error at all, but just a way to stop the script and output some debugging info for you to manually parse. The second one is not an error per se, but will be converted into an error if you don't catch it. The last one is triggering a real error in the PHP engine which will be handled according to the configuration of your PHP environment in some cases shown to the user, in other cases just logged to a file or not saved at all.
Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. Asked 10 years, 3 months ago. Active 4 years, 7 months ago. Viewed 32k times. Add error-checking code whenever you can predict that an error might occur if your code gets bad input data.
Expect that at some time, your code will be passed incorrect or null parameters, that your queries will return no rows or more rows than you expect. Make your programs robust enough to work even if the database is not in the state you expect. For example, perhaps a table you query will have columns added or deleted, or their types changed. Learn the names and causes of the predefined exceptions.
If your database operations might cause particular ORA- errors, associate names with these errors so you can write handlers for them. You will learn how to do that later in this chapter. Write out debugging information in your exception handlers. You might store such information in a separate table. Carefully consider whether each exception handler should commit the transaction, roll it back, or let it continue.
Remember, no matter how severe the error is, you want to leave the database in a consistent state and avoid storing any bad data. Using exceptions for error handling has several advantages.
With exceptions, you can reliably handle potential errors from many statements with a single exception handler:. If the exception is ever raised in that block or any sub-block , you can be sure it will be handled. Sometimes the error is not immediately obvious, and could not be detected until later when you perform calculations using bad data. Again, a single exception handler can trap all division-by-zero errors, bad array subscripts, and so on. If you need to check for errors at a specific spot, you can enclose a single statement or a group of statements inside its own BEGIN-END block with its own exception handler.
You can make the checking as general or as precise as you like. You need not declare them yourself. You can write handlers for predefined exceptions using the names in the following table:. Because this exception is used internally by some SQL functions to signal completion, you should not rely on this exception being propagated if you raise it within a function that is called as part of a query. Unlike predefined exceptions, user-defined exceptions must be declared and must be raised explicitly by RAISE statements.
Exception and variable declarations are similar. But remember, an exception is an error condition, not a data item. Unlike variables, exceptions cannot appear in assignment statements or SQL statements. However, the same scope rules apply to variables and exceptions. You cannot declare an exception twice in the same block.
You can, however, declare the same exception in two different blocks. Exceptions declared in a block are considered local to that block and global to all its sub-blocks. Because a block can reference only local or global exceptions, enclosing blocks cannot reference exceptions declared in a sub-block.
If you redeclare a global exception in a sub-block, the local declaration prevails. The sub-block cannot reference the global exception, unless the exception is declared in a labeled block and you qualify its name with the block label:.
To have the enclosing block handle the raised exception, you must remove its declaration from the sub-block or define an OTHERS handler. A pragma is a compiler directive that is processed at compile time, not at run time. That lets you refer to any internal exception by name and to write a specific handler for it.
When you see an error stack, or sequence of error messages, the one on top is the one that you can trap and handle. The pragma must appear somewhere after the exception declaration in the same declarative section, as shown in Example That way, you can report errors to your application and avoid returning unhandled exceptions. If the optional third parameter is TRUE , the error is placed on the stack of previous errors. This function is useful when you need to generate a particular response to an exception at runtime.
It's limited to bytes in length. Any additional characters beyond bytes will be truncated. The designated error type for this error. HTML entities in message are not escaped. Use htmlentities on the message if the error is to be displayed in a browser.
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