TypeScript function to extract message from an error of unknown type, useful in catch blocks UNTESTED

parse-caught-error.ts

/**
 * Detect type of caught error and return its message, optionally running type-specific callbacks
 *
 * @example
 * catch (err) {
 *   const { message } = parseCaughtError(err);
 *   console.log(message);
 * }
 *
 * @NOTE Returns an option, to ease iterating to return other properties when needed e.g. stack
 *
 * @param error Error of unknown type to parse
 * @param cbs   Optional object with callbacks to invoke depending on detected type
 *
 * @return Object with extracted properties from the error
 */
function parseCaughtError(
  error: unknown,
  cbs?: {
    typeString?: (error: string) => void;
    // tslint:disable-next-line:ban-types
    typeStringClass?: (error: String, message: string) => void;
    typeErrorClass?: (error: Error, message: string) => void;
    typeObject?: (error: object, message: string) => void;
  }
): { message: string } {
  let message: string | undefined = 'Unknown error occurred';

  if (error !== null && error !== undefined) {
    if (typeof error === 'string') {
      message = error;
      cbs?.typeString !== undefined && cbs.typeString(error);
    } else if (error instanceof String) {
      message = error.toString();
      cbs?.typeStringClass !== undefined && cbs.typeStringClass(error, message);
    } else if (error instanceof Error) {
      message = error.message;
      cbs?.typeErrorClass !== undefined && cbs.typeErrorClass(error, message);
    } else if (typeof error === 'object') {
      const errObj = error as {
        [prop: string]: { toString: () => string };
      };
      message = errObj.message?.toString();
      cbs?.typeObject !== undefined && cbs.typeObject(error, message);
    }
  }

  return { message };
}

Iterates on some work from Dan Story.

Untested.