Custom error param

[tuntag]

My project needs to return a custom error format including the error param/key, so please consider to add this feature.

This new additional method could be .withParam() similar to the .withMessage()

Examples

body("name")
  .custom(val => {...check duplicated })
  .withMessage("name is duplicated")

this returns { "name" : "string only" }

Expected

body("name")
  .custom(val => {...check duplicated })
  .withParam("name_exists") // request func
  .withMessage("name is duplicated")

this returns { "name_exists" : "string only" }

[fedeci]

I'm not sure of why you would like to change the param name that failed. withMessage accepts an object as a param, so the right place to perform this kind of error response should be there.
e.g.

.withMessage({ reason: "name_esists", message: "name is duplicated" })

[tuntag]

Hi, I know this doesn't good practice but you know that we could meet some projects that just need an array of errors key, such as mapping their i18n keys like: for(key in keys) return errors[key]

I have rewrite the result as my needs:

const format = (e: ValidationError): Result<any> => {
    return typeof e.msg === "object"
      ? e.msg
      : e.nestedErrors
        ? e.msg !== "Invalid value(s)"
          ? { require_one: e.msg } // show error message if not default message
          : { require_one: e.nestedErrors.reduce((o, er) => Object.assign(o, this.format(er)), {} as ValidationError) }
        : { [e.param]: e.msg };
}
const result = validationResult(this.req).formatWith(e => format(e));

...but it's not good at all.

Everything would be great if this provides a simple method to change it independently.