Currently, the default error handler in koa checks for the status property on errors and sets the status accordingly. However, many errors in the HTTP spec are defined with required or recommended headers to be send along with the status. For example, 405 Method Not Allowed should always be sent with an Allow header, a 503 Service Unavailable with a Retry After.
Currently, it is not possible to set this. You could write your own error handler that checks a headers property on the error object, but third party middleware can't rely on that. As a result, middleware modules on npm (like koa-router etc.) have to set status and headers on the context response manually instead of throwing an error, which means it is not possible to catch them and therefor destroys one of the best features of koa - error handling with try / catch.
It would be nice if the koa error handler would check for the existence of a headers property on the object and set the headers in that hash (with normalized casing). http-errors already supports passing a properties hash in the constructor, so it could look like this:
throw new HttpError(405, {headers: {'Allow': 'GET, POST'}})
throw new HttpError(503, {headers: {'Retry-After': 60}})
This would then allow middlewares from npm to actually throw errors that can be catched to add some additional custom text etc.
Thoughts?
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