45 lines
1.6 KiB
Markdown
45 lines
1.6 KiB
Markdown
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# Error Handling
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## Try-Catch
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Using generators means that you can try-catch `next`. For example,
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this example prepends all error messages with "Error: "
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```js
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app.use(function*(next){
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try {
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yield next;
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} catch (error) {
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error.message = 'Error: ' + error.message;
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throw error;
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}
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});
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```
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### Default Error Handler
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The default error handler is essentially a try-catch at
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the very beginning of the middleware chain. To use a
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different error handler, simply put another try-catch at
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the beginning of the middleware chain, and handle the error
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there. However, the default error handler is good enough for
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most use cases. It will use a status code of `err.status`,
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or by default 500. If `err.expose` is true, then `err.message`
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will be the reply. Otherwise, a message generated from the
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error code will be used (e.g. for the code 500 the message
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"Internal Server Error" will be used). All headers will be
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cleared from the request, but any headers in `err.headers`
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will then be set. You can use a try-catch, as specified
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above, to add a header to this list.
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## The Error Event
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Error handlers can be specified with `app.on('error')`.
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If no error handler is specified, a default error handler
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is used. Error handlers recieve all errors that make their
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way back through the middleware chain, if an error is caught
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and not thrown again, it will not be handled by the error
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handler. If not error event handler is specified, then
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`app.onerror` will be used, which simply log the error if
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`error.expose` is true and `app.silent` is false.
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