[SirensOfTitan]

One control flow function I use often in JavaScript is my `runIfUnhandled` function:

    export const UNHANDLED: unique symbol = Symbol("UNHANDLED");
    export function runIfUnhandled<TReturn = void>(
      ifUnhandled: () => TReturn,
      run: (unhandled: Unhandled) => TReturn | Unhandled,
    ): TReturn {
      const runResult = run(UNHANDLED);
      if (runResult === UNHANDLED) {
        return ifUnhandled();
      }
      return runResult;
    }

I often use it to use guards to bail early, while keeping the default path dry. In particular, I used it heavily in a custom editor I built for a prior project, example:

        runIfUnhandled(
          () => serialize(node, format),
          (UNHANDLED) => {
            if (
              !Element.isElement(node) ||
              !queries.isBlockquoteElement(node)
            ) {
              return UNHANDLED;
            }

            switch (format) {
              case EditorDocumentFormat.Markdown: {
                const serialized = serialize(node, format) as string;
                return `> ${serialized}`;
              }
              default: {
                return UNHANDLED;
              }
            }
          },
        );