How to add custom paramater in error object lb4?

Question

I am using strongloop 4 (lb4). I am facing one issue that in error object I need to one more custom parameter in the error object.

I want it on the global level. On every error, I want to add that custom parameter in every error message.

In loopback4 global error handling is done by src/sequence.ts.

Suppose the error message object is.

{
  "error": {
    "statusCode": 400,
    "name": "xyz",
    "message": "firstName is required"
  }
}

I want error object output like.

{
  "error": {
    "customParam" : "customParam",
    "statusCode": 400,
    "name": "xyz",
    "message": "firstName is required"
  }
}

Answer 1

I tried adding error object into new object.

                       let error = new Error();
                        error.name = 'Invalid_OTP_AttemptsError';
                        error.status = 422;
                        error.message = 'You’ve exceeded the maximum number of One-Time Password (OTP) attempts';
                        let data={...error};
                        data.retryCount=foundMb.retryCount
                        data.resendCount=foundMb.resendCount
                        return callback(null,data);

Answer 2

Cross-posting the answer I gave on GitHub in https://github.com/strongloop/loopback-next/issues/1867#issuecomment-434247807

Building HTTP error responses is a tricky business. It's easy to get it wrong and open your application to attacks.

In LoopBack (both 3.x and 4.x), we use our strong-error-handler middleware to take care of this. See Handling Errors in our docs.

Here are the important security constraints to keep in mind:

In production mode, strong-error-handler omits details from error responses to prevent leaking sensitive information:

• For 5xx errors, the output contains only the status code and the status name from the HTTP specification.
• For 4xx errors, the output contains the full error message (error.message) and the contents of the details property (error.details) that ValidationError typically uses to provide machine-readable details about validation problems. It also includes error.code to allow a machine-readable error code to be passed through which could be used, for example, for translation.

In debug mode, strong-error-handler returns full error stack traces and internal details of any error objects to the client in the HTTP responses.

Now that I have warned you, LoopBack 4 makes it very easy to format the error messages your way. Just provide a custom implementation of the Sequence action reject. See Customizing Sequence Actions in our docs, it explain how to create a custom send action. The solution for reject is pretty much the same, you just need a different signature for the action function.

export class CustomRejectProvider implements Provider<Reject> {
  // ...
  action({request, response}: HandlerContext, error: Error) {
    // handle the error and send back the error response
    // "response" is an Express Response object
  }
}

Caveat: some errors thrown by LB4 have only code set, these errors need a bit of pre-processing to decide what HTTP status code they should trigger. (For example, the error code ENTITY_NOT_FOUND should be mapped to the status code 404). The built-in reject action does not yet expose this pre-processing for consumption by custom reject actions. It's an oversight on our side, l created a new issue https://github.com/strongloop/loopback-next/issues/1942 to keep track of that.