feat(example-validation-app): customize error

[dhmlau]

Add code for customizing error to validation-app example.

Will add docs after #4874 lands.

Checklist

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• npm test passes on your machine
• New tests added or existing tests modified to cover all changes
• Code conforms with the style guide
• API Documentation in code was updated
• Documentation in /docs/site was updated
• Affected artifact templates in packages/cli were updated
• Affected example projects in examples/* were updated

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[raymondfeng]

CI fails due to code formatting issues. Run npm run prettier:fix to fix it.

[raymondfeng]

@dhmlau Please fix https://travis-ci.com/github/strongloop/loopback-next/jobs/303320121 as follows:

cd examples/validation-app
rm -rf node_modules package-lock.json
cd ../..
npm i

[bajtos]

This isn't the best way how to present an error in the response. I believe this is going to drop the stack trace, which will make debugging difficult? At the same time, we don't want to print stack traces in production. All of this is handled by our default reject action. Can you please update this example to show how the custom error handler can fall back to the default implementation?

[emonddr]

@dhmlau , in our shopping example sequence, we catch an error like this : https://github.com/strongloop/loopback4-example-shopping/blob/master/packages/shopping/src/sequence.ts#L72 inside the sequence, and pass them to default reject action .

[dhmlau]

@emonddr, thanks for pointing to the code snippet!

[emonddr]

Not sure if @bajtos is asking to handle the error like authentication sequence handles it.

[emonddr]

@dhmlau , in our shopping example sequence, we catch an error like this : https://github.com/strongloop/loopback4-example-shopping/blob/master/packages/shopping/src/sequence.ts#L72 inside the sequence, and pass them to default reject action .

[dhmlau]

@emonddr @bajtos, for falling back to the default reject action, I used RejectProvider. Please see my latest commit 76594bd. Thanks.

[nabdelgadir]

Would it make sense to link to https://loopback.io/doc/en/lb4/Creating-components.html#providers instead?

[bajtos]

I was on a call with few IBMers interested in custom error handling and it occurred to me that the solution based on a custom Reject implementation may be too complex. Unless the developer want to share the custom error handling, then I think it's much easier to implement the custom path inside the sequence (src/sequence.ts in projects scaffolded via lb4).

export class MySequence implements SequenceHandler {
  // ...
  async handle(context: RequestContext) {
    try {
      // ...
    } catch (error) {
      const httpError = <HttpErrors.HttpError>error;
      if (request.url === '/coffee-shops') {
        // if this is a validation error
        if (httpError.statusCode === 422) {
          const newError = {
            message: 'My customized validation error message',
            code: 'VALIDATION_FAILED',
            resolution: 'Contact your admin for troubleshooting.',
          };

          // you can change the status code here too
          response.status(422).send(JSON.stringify(newError));

          // TODO: show how to log the error using RestBindings.SequenceActions.LOG_ERROR

          // The error was handled
          return;
        }
      }

      // Otherwise fall back to the default error handler
      this.reject(context, error);
    }
  }
}

Just an idea to consider.

[bajtos]

For longer term, I think it would be best to implement support for a custom JSON serializer in strong-error-handler, see loopbackio/strong-error-handler#8. This will allow app developers to simply change RestBindings.ERROR_WRITER_OPTIONS to provide a custom function that will be called by LB to convert error objects into HTTP response bodies.

[dhmlau]

Thanks @bajtos for the code snippet. I was trying to figure out how to fall back to the default implementation. :).
I've updated accordingly to your review comments in 082113e, PTAL again, @nabdelgadir @bajtos. Thanks!

[bajtos]

I find the new version with the custom error handling code inside the sequence as easier to understand and more elegant 👍

However, it seems that implementation details in my suggestion have confused you a bit, sorry for that! 🙈 Please see my comments below for more explanation, hopefully it will be all clear now 🤞

[dhmlau]

@bajtos, please see my last commit a5ced91 on the changes. Thanks!

[bajtos]

Great 👏