Typescript not resolving `<T extends Error>`

Within my interface definition, I have the following:

export interface IErrorIdentification {
  errorClass?: new <T extends Error>() => T;
  code?: string;
  name?: string;
  messageContains?: string;
}

However, I am facing issues with the errorClass property. When I attempt to use it in this way:

context.errorMeta.add(
  404,
  { errorClass: HandledError },
  { callback: e => cbResult }
);

The problem arises when the second parameter, { errorClass: HandledError }, is supposed to adhere to the IErrorIdentificaiton type. This results in the following error:

https://i.sstatic.net/WwIQM.png

Interestingly, things run smoothly during runtime with this check:

e instanceof i.identifiedBy.errorClass

In addition, the error (e) is recognized as an instance of Error, which aligns with the fact that

HandledError</co de> is defined like so:</p>

<pre class="lang-js"><code>export class HandledError extends Error { ... }

Despite all these factors, I'm puzzled by the error message and its lack of clarity. Can anyone help me identify where I may be going wrong?

Answer №1

errorClass?: new <T extends Error>() => T;

The interpretation might not be what you initially thought. It is stating that "errorClass should be a generic entity on which the new operation can be performed, resulting in any specified error type." In other words, for every T that extends Error, invoking new should return a corresponding T. The existing structure of HandledError does not fulfill this requirement as it cannot produce various error types like

TypeError</code. As far as I'm aware, it's uncertain whether Typescript has a mechanism to generate a (non-<code>any
) value that complies with this constraint.

If your objective is to allow for the existence of a T where new yields a T, you are describing an existential type, which is not supported by Typescript. Consequently, you have two potential solutions. If you are indifferent to the specific error type, simplify the setup and have new simply return Error.

export interface IErrorIdentification {
  errorClass?: new() => Error;
}

Conversely, if you genuinely require the T value for particular scenarios, introduce parameters into your interface.

export interface IErrorIdentification<T extends Error> {
  errorClass?: new() => T;
}

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