denoland/deno · error · TypeError

Expected 'fn' field in the first argument to be a bench func

Error message

Expected 'fn' field in the first argument to be a bench function

What it means

`deno desktop --hmr` spawns the project's dev server (e.g. Vite) and scans its stdout for a `Local: http://…` line. This error means the server process exited before printing any such line, so no URL was ever obtained. The server's own crash output is echoed above via log::info.

Source

Thrown at cli/js/40_bench.js:171

      fn: nameOrFnOrOptions,
      name: nameOrFnOrOptions.name,
    };
  } else {
    let fn;
    let name;
    if (typeof optionsOrFn === "function") {
      fn = optionsOrFn;
      if (nameOrFnOrOptions.fn != undefined) {
        throw new TypeError(
          "Unexpected 'fn' field in options, bench function is already provided as the second argument",
        );
      }
      name = nameOrFnOrOptions.name ?? fn.name;
    } else {
      if (
        !nameOrFnOrOptions.fn || typeof nameOrFnOrOptions.fn !== "function"
      ) {
        throw new TypeError(
          "Expected 'fn' field in the first argument to be a bench function",
        );
      }
      fn = nameOrFnOrOptions.fn;
      name = nameOrFnOrOptions.name ?? fn.name;
    }
    if (!name) {
      throw new TypeError("The bench name can't be empty");
    }
    benchDesc = { ...defaults, ...nameOrFnOrOptions, fn, name };
  }

  const AsyncFunction = (async () => {}).constructor;
  benchDesc.async = AsyncFunction === benchDesc.fn.constructor;
  benchDesc.fn = wrapBenchmark(benchDesc);
  benchDesc.warmup = false;
  benchDesc.name = escapeName(benchDesc.name);
  if (cachedOrigin == undefined) {

View on GitHub (pinned to 89f33cbef2)

Solutions

1. Run the dev server command manually (e.g. `npm run dev`) and fix its startup error — the crash reason is the server's own output
2. Install dependencies first (`npm install` or `deno install`) before `deno desktop --hmr`
3. Free the occupied port or configure the server to use another one
4. Check the toolchain version (Node/npm) matches the project's requirements

Example fix

# before
deno desktop --hmr   # dev server exits silently

# after
npm run dev            # reproduce and fix the server's error (install deps, free port)
deno desktop --hmr

const portFree = await probeTcp('127.0.0.1', 5173) === 'closed';
if (!portFree) throw new Error('dev port busy — stop the other server first');
await run('npm', ['install']); // deps present so the server reaches its Local: line

try {
  await denoDesktopHmr();
} catch (e) {
  if (/exited without printing a URL/.test(String(e))) {
    const log = await runRaw('npm', ['run', 'dev']); // server's own crash output names the cause
    throw new Error(`dev server crashed: ${log}`);
  }
  throw e;
}

When it happens

Trigger: The dev server dies at startup: missing node_modules, port already in use, syntax/config error in vite.config.ts, or an unsupported Node version — and exits during the URL scan loop.

Common situations: First run without installing dependencies; another process holding the dev port; a broken vite/webpack config; CI environments where the dev server needs setup the script did not do.

Related errors

• Unexpected second argument to Deno.bench()
• The bench name can't be empty
• Unexpected 'name' field in options, bench name is already pr
• The bench function must have a name
• Unexpected third argument to Deno.bench()

AI-assisted analysis of denoland/deno@89f33cbef2 (2026-08-16). Data as JSON: /api/errors/3ffceb29a5b7ee55. Report an issue: GitHub.