Loopback-next: Customizing error messages

[jangeorgiev]

Feature proposal

I have noticed that currently there are no way of customizing error messages returned from AJV in LB4.
I have found that there is implementation of handling error message already - AJV-Errors.
I'll be very happy to be able to customize error messages, because some of them are extremely ugly.

Example:

See code here

The code above in case of violation of patter will return
"should match pattern \"^[a-z]+$\""

[shadyanwar]

Related to https://github.com/strongloop/loopback-next/issues/1867

[dhmlau]

@jangeorgiev , is #1867 that @shadyanwar mentioned above something you're looking for? If yes, I'd like to continue the discussion over there instead. Thanks.

[jangeorgiev]

No, it's not related to custom errors that are returned from HttpErrors.UnprocessableEntity().
I talk about customizing error messages of predefined validation rules see all rules here

[TomerSalton]

@YaelGit might be relevant to you?

[bajtos]

Based on the issue description and the discussion, I see two incremental steps we can make:

1. Change our validation layer to use ajv-errors to produce the error message, instead of relying on the built-in ajv messages.

2. Create an extensibility point allowing LB4 applications to further customize the error messages.

3. Probably out of scope of this issue, just something to keep in consideration: localize error messages to a different langauge based on server ENV variables and/or request headers.

Does it look like a reasonable plan to you?

I don't think this will become a priority for us in the next 3-6 months, so it's up to you to contribute these improvements. I am happy to help you along the way if you decide so.

See https://loopback.io/doc/en/contrib/code-contrib.html to get started.

[shadyanwar]

Change our validation layer to use ajv-errors to produce the error message, instead of relying on the built-in ajv messages.

@bajtos I hope to find time next weekend to work on this. It would be nice if you tell me where to start.

[bajtos]

@shadyanwar that's great to hear!

Here is the code converting AJV validation errors to an HttpError instance:

https://github.com/strongloop/loopback-next/blob/27b6accace55761d8db15442e5ca7705bab7d9fc/packages/rest/src/validation/request-body.validator.ts#L128-L137

[AaronJan]

Hi @bajtos, I made a PR (#3746 ), advice needed!

[dhmlau]

@jangeorgiev, is this issue good to close? Thanks.

[jangeorgiev]

Hey @dhmlau, thanks for asking.

I updated my project to last version and I tried to use custom error messages as described in AJV Documentation, but it didn't worked.
Should I install some additional packages (i.e. "ajv-i18n") ?
Actually, could you or @AaronJan please show me an example how to do it in your way ?

Thanks in advance.

[AaronJan]

@jangeorgiev 😄 No problem!

You have to install ajv-i18n first, then put this code in your Application:

const localize = require('ajv-i18n');

export class MyApplication {
  constructor() {
  // ...

  this.bind(RestBindings.REQUEST_BODY_PARSER_OPTIONS).to({
      validation: {
        ajvErrorTransformer: (errors: AJV.ErrorObject[]): AJV.ErrorObject[] => {
          // `.zh()` is for Chinese, for other languages check here: https://github.com/epoberezkin/ajv-i18n#supported-locales
          localize.zh(errors);

          // Because the translation function is mutable, you have to return the original object.
          return errors;
        },
      },
    });
  }
}

[jangeorgiev]

Great! The example works perfect!

Still I have no idea how to integrate ajv-errors, but it's not big deal for me, I'll just write work-around thanks to ajvErrorTransformer.

Thanks for hard work guys !