E0296: abstract properties are only allowed in abstract classes

TypeScript abstract classes can contain abstract methods and fields. It is a syntax error to write an abstract method or field in a class not marked abstract:

class Logger {
  // Implement this in subclasses.
  abstract log(
    severity: LogLevel,
    message: string,
  );
}

class ConsoleLogger extends Logger {
  log(
    severity: LogLevel,
    message: string,
  ) {
    console.log(`${severity}: ${message}`);
  }
}

To fix this error, add the abstract keyword to the class:

abstract class Logger {
  // Implement this in subclasses.
  abstract log(
    severity: LogLevel,
    message: string,
  );
}

Alternatively, make the method non-abstract by removing the abstract keyword and giving it a body:

abstract class Logger {
  // Optionally implement this in subclasses.
  log(
    severity: LogLevel,
    message: string,
  ) {
    // Default behavior: do not log.
  }
}

Introduced in quick-lint-js version 2.10.0.

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