onFilter returning error details rather than global "disallowed info_hash"

[lorentzkim]

Since filter function has become async and receives a callback, perhaps it'll be good to have functionality for standard callback(err, res) form which allows for errors to be passed through, for customised error reasons.

It'll be more usable than just having "disallowed info_hash" for every reason for failure.

[feross]

I was trying to make the interface like Array.prototype.filter as much as possible.

I agree, it would be nice to have a way to specify a message. But the standard callback(err, res) interface doesn't make much sense. What is res?

We could do just callback(err). If err == null, then it's allowed. If err is truthy, then it's disallowed and if err instanceof Error then the custom error message will be used. But this is another API change...

[feross]

Okay, I found a solution I like. It's not a breaking API change.

In addition to returning a boolean (true for allowed, false for disallowed), you can return an Error object to disallow and provide a custom reason.

Released as 5.1.0.

[lorentzkim]

Thank you! That solution works.

My reference to err/res was about this: http://thenodeway.io/posts/understanding-error-first-callbacks/

I thought it was convention that async functions return function(err, res), where err is the error object (if exists, null otherwise), and res for result, whatever it may be.

Of course, in this case, "res" wouldn't be used. It'll just be an Error instance, if it's there.

Maybe for next API break when there's a time for it, if you agree with the convention.