github-desktop/docs/technical/error-reporting.md

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# Error Reporting
First we need to make the distinction between expected runtime _errors_ and
_exceptions_. Unfortunately both are represented with the `Error` class, but
they're conceptually different. Exceptions are fatal, errors are not.
The story around exceptions is simpler so let's start there.
## Exceptions
An exception is an unexpected, fatal problem in the app itself. For example, our
old friend `undefined is not a function`. This is a problem with the code itself
which cannot be resolved at runtime. Our only option is to quit the app and
relaunch.
We handle uncaught exceptions by registering a [global listener](https://github.com/desktop/desktop/blob/fb4e73560127f491ccf5f59984a310481911f2b6/app/src/ui/index.tsx#L75).
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We report the exception to Central, tell the user that an unrecoverable error
happened, and then quit and relaunch. End of story.
## Errors
Errors are a bit more involved. They are anything that can go wrong in the
standard usage of the app. For example, if the internet's down or a git
repository is in a funny state, we're gonna get some errors.
Our error reporting flows through the `Dispatcher` like most everything in the
app. [`postError`](https://github.com/desktop/desktop/blob/fb4e73560127f491ccf5f59984a310481911f2b6/app/src/lib/dispatcher/dispatcher.ts#L308)
calls the [registered](https://github.com/desktop/desktop/blob/fb4e73560127f491ccf5f59984a310481911f2b6/app/src/lib/dispatcher/dispatcher.ts#L711)
[error handlers](https://github.com/desktop/desktop/blob/fb4e73560127f491ccf5f59984a310481911f2b6/app/src/lib/dispatcher/error-handlers.ts),
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starting with the most recently registered. The error handlers have the chance
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to pass the error through untouched, return a different or more specific error,
or swallow the error entirely.
Error handlers must have the following type:
```typescript
export async function myCoolErrorHandler(
error: Error,
dispatcher: Dispatcher
): Promise<Error | null> {
// code goes here
}
```
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If an error passes through all the registered error handlers, the final error
handler will call [`Dispatcher#presentError`](https://github.com/desktop/desktop/blob/75445ea61177347b2df08e846aae30e637d5f1de/app/src/lib/dispatcher/dispatcher.ts#L334).
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That will present the generic error dialog to the user.
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```
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+------------------------+
| |
| Dispatcher#postError |
| |
+------------------------+
|
|
+------------------+ +--------------------+
| | | |
| error handlers |-----| do something else |
| | | |
+------------------+ +--------------------+
|
|
+-------------------------+
| |
| Dispatcher#presentError |
| |
+-------------------------+
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```
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### Subclasses
We have a couple `Error` subclasses which we use to provide more context to
error handlers:
* [`GitError`](https://github.com/desktop/desktop/blob/75445ea61177347b2df08e846aae30e637d5f1de/app/src/lib/git/core.ts#L62) -
All errors coming from git should be `GitError`s.
* [`ErrorWithMetadata`](https://github.com/desktop/desktop/blob/75445ea61177347b2df08e846aae30e637d5f1de/app/src/lib/error-with-metadata.ts) -
Wraps an existing `Error` with additional metadata.
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`ErrorWithMetadata` instances can provide a [`RetryAction`](https://github.com/desktop/desktop/blob/75445ea61177347b2df08e846aae30e637d5f1de/app/src/lib/retry-actions.ts)
which gives error handlers the ability to retry whatever action caused the error.