(JS) Expose handler function type to allow manual exporting of Lambda handler

[hugo-vrijswijk]

Related to #246

Needs some more docs, maybe some further (manual) testing, and possibly an entirely different implementation. But I wanted to open this PR to have an example and get the discussion flowing.

With this, you can implement and export your lambda as follows:

object Lambda extends IOLambda[String, String] {

  override def handler: Resource[IO, LambdaEnv[IO, String] => IO[Option[String]]] = ???

  @JSExportTopLevel("handler")
  def impl: HandlerFn = handlerFn
}

[hugo-vrijswijk]

If you're really brave I guess ideally we would have some integration tests against the actual AWS JS runtime that verify it works for both CommonJS and ESModules.

I have no idea how I'd even start on this 😅

[armanbilge]

I have no idea how I'd even start on this

If you want to do this 😁 start by making a new project with feral and figure out how to run it against SAM locally. Then, write those commands into an sbt task. Then, we make that into a scripted test.

Some of my old work here might be helpful: https://github.com/ChristopherDavenport/js-test/tree/serverless

Anyway just driving by, sorry, will come back later with another review pass :) thanks for all your work!

[armanbilge]

I wonder if instead of dealing with SAM it would be easier to work with the Node.js runtime directly. https://www.npmjs.com/package/aws-lambda-ric https://github.com/aws/aws-lambda-nodejs-runtime-interface-client

[armanbilge]

Cool, I did some investigation. It seems the trick is to install aws-lambda-ric and and aws-lambda-rie.

Then we start the lambda like this:

$ aws-lambda-rie aws-lambda-ric app.handler

where app.js contains

exports.handler = async (event, context) => {
  return 'Hello World!';
}

which can be invoked like:

$ curl -XPOST "http://localhost:8080/2015-03-31/functions/function/invocations" -d '{}'
"Hello World!"

No pressure to take this on :) but given my previous hiccups with exports I would like to add this integration test just to be sure that everything works.

[armanbilge]

Hmm, this is confusing.

• https://github.com/aws/aws-lambda-nodejs-runtime-interface-client/issues/44

[armanbilge]

Ah, instead of installing the ric and rie seems we can directly invoke the docker image. https://hub.docker.com/r/amazon/aws-lambda-nodejs