How to get an instance type from generic type?

I have the following classes:

export class ValidationFailedError extends Error {
  public errors: ValidationError[];

  constructor(errors: ValidationError[]) {
    super('Validation errors exist');

    this.errors = errors;
  }
}

export class ValidationError {}
export class DuplicateFileValidationError extends ValidationError {}
export class MultipleTransactionTypesValidationError extends ValidationError {}
export class FileIsPlacedInWrongFolderValidationError extends ValidationError {
  constructor(public fileName: string, public expectedFolder: string, public actualFolder: string) {
    super();
  }
}
export class InvalidTransactionsCountValidationError extends ValidationError {
  constructor(
    public transactionsCount: number,
    public minTransactionsCount: number,
    public maxTransactionsCount: number,
  ) {
    super();
  }
}

And I have some tests (I'm using Jest) that check if function throws a ValidationFailedError, and then also checks, what is inside ValidationFailedError.errors array. For the tests I wrote the following helper function which checks that ValidationFailedError error contains only one error of provided ValidationError, and performs some custom asserts provided as a callback function which takes single argument - ValidationError with expected type as an argument:

function expectValidationFailedErrorContainsSingleError<T extends ValidationError>(
  validationFailedError: ValidationFailedError,
  errorCtor: T,
  customAssertAction?: (err: T) => void,
): void {
  const { errors } = validationFailedError;
  expect(Array.isArray(errors)).toBe(true);
  expect(errors.length).toEqual(1);

  const validationError = errors[0] as T;
  expect(validationError).toBeInstanceOf(errorCtor);

  customAssertAction(validationError);
}

The function above is compiled fine, however I have an issue with using this function in my tests. Please take a look at the example below, I took this piece of code from one of my tests:

expectValidationFailedErrorContainsSingleError(
  err,
  FileIsPlacedInWrongFolderValidationError,
  (validationError) => {
    expect(validationError.fileName).toEqual('test-file.json');
    expect(validationError.actualFolder).toEqual(actualFolder);
    expect(validationError.expectedFolder).toEqual(expectedFolder);
  },
);

Typescript does not recognize the type of validationError parameter, it raises the following 3 errors:

> Property 'fileName' does not exist on type 'typeof
> FileIsPlacedInWrongFolderValidationError'.ts(2339)
>
> Property 'actualFolder' does not exist on type 'typeof
> FileIsPlacedInWrongFolderValidationError'.ts(2339)
>
> Property 'expectedFolder' does not exist on type 'typeof
> FileIsPlacedInWrongFolderValidationError'.ts(2339)

When I hover on validationError parameter, I see that the type of this parameter is typeof FileIsPlacedInWrongFolderValidationError :

I feel like the type of validationError is not an instance type, it's a class type.

The Question: Is it possible to fix this issue, so I can access the properties fileName, actualFolder and expectedFolder in the function?

Assuming you don't plan on calling the errorCtor (since you can't know if or how many arguments it has here), this typing doesn't show any warnings on TS Playground:

function expectValidationFailedErrorContainsSingleError<T extends ValidationError>(
  validationFailedError: ValidationFailedError,
  errorCtor: new (...args: never[]) => T, // Constructor type
  customAssertAction?: (err: T) => void,
): void {
  const { errors } = validationFailedError;
  expect(Array.isArray(errors)).toBe(true);
  expect(errors.length).toEqual(1);

  const validationError = errors[0] as T;
  expect(validationError).toBeInstanceOf(errorCtor);

  customAssertAction?.(validationError); // Note the optional chaining
}

const err = new ValidationFailedError([
  new FileIsPlacedInWrongFolderValidationError('test-file.json', '/etc/ts-test', '/etc/ts_test')
])
expectValidationFailedErrorContainsSingleError(
  err,
  FileIsPlacedInWrongFolderValidationError,
  (validationError) => {
    const { fileName, actualFolder, expectedFolder } = validationError
    expect(fileName).toEqual('test-file.json');
    expect(actualFolder).toEqual('/etc/ts-test');
    expect(expectedFolder).toEqual('/etc/ts_test');
  },
);