What causes "Invalid type for fetch: must be a function" in denoland/deno?

Make the exported fetch a function: export const fetch = (req: Request) => new Response('ok'); or export default { fetch(req) { ... } }.

denoland/deno · error · TypeError

Invalid type for fetch: must be a function

Error message

Invalid type for fetch: must be a function

What it means

Thrown by Deno's declarative server registration (registerDeclarativeServer), which runs when a module exports a top-level 'fetch' export (the module-as-server style used by 'deno serve' and deploy-style entrypoints). If the exports object has an own property 'fetch', that value must be a function; anything else (a number, string, object, class instance) is rejected before Deno.serve is created.

Source

Thrown at ext/http/00_serve.ts:1640

    },
    [SymbolAsyncDispose]() {
      return this.shutdown();
    },
  };
}

internals.addTrailers = addTrailers;
internals.serveHttpOnListener = serveHttpOnListener;
internals.serveHttpOnConnection = serveHttpOnConnection;
internals.resetLegacyAbortWarning = () => {
  legacyAbortWarned = false;
};

function registerDeclarativeServer(exports) {
  if (!ObjectHasOwn(exports, "fetch")) return;

  if (typeof exports.fetch !== "function") {
    throw new TypeError("Invalid type for fetch: must be a function");
  }

  if (
    exports.onListen !== undefined && typeof exports.onListen !== "function"
  ) {
    throw new TypeError("Invalid type for onListen: must be a function");
  }

  return ({
    servePort,
    serveHost,
    workerCountWhenMain,
  }) => {
    const server = Deno.serve({
      port: servePort,
      hostname: serveHost,
      [kLoadBalanced]: workerCountWhenMain == null
        ? true

View on GitHub (pinned to 89f33cbef2)

Solutions

Make the exported fetch a function: export const fetch = (req: Request) => new Response('ok'); or export default { fetch(req) { ... } }.
If 'fetch' is not meant to be a server handler, rename the export (e.g. fetchConfig) so the module does not look like a declarative server.
Run the file with 'deno run' plus an explicit Deno.serve(handler) call instead of relying on the export-based convention.

Example fix

// before
export const fetch = { handler: (req) => new Response("hi") }; // object, not callable

// after
export default {
  fetch(req: Request) {
    return new Response("hi");
  },
};

Defensive patterns

Strategy: type-guard

Type guard

type ServeExports = { fetch?: (req: Request, info: { remoteAddr; url }) => Response | Promise<Response>; onListen?: (info: { hostname: string; port: number }) => void };
function isValidServeModule(m: ServeExports): boolean { return typeof m.fetch !== "function" ? false : true; }

Prevention

Type entrypoint exports explicitly instead of relying on inference.
Never name non-handler exports 'fetch' in serve entrypoints.
Run `deno check` on the entry module in CI to catch non-function exports.

When it happens

Trigger: A main/entry module run via 'deno serve' (or a worker spawned for it) whose default or named export object contains fetch: 42, fetch: 'handler', or fetch bound to a non-callable; accidentally exporting a variable named fetch that shadows the global fetch builtin; export const fetch = someObject.handler missing the .bind.

Common situations: Naming a data field or constant 'fetch' in a module that is also used as a serve entrypoint; refactoring a Request handler into a config object and exporting the object; copy-pasting deploy example code into a module that already exports an unrelated fetch symbol.

Related errors

AI-assisted analysis of denoland/deno@89f33cbef2 (2026-08-16). Data as JSON: /api/errors/8e5eaf2c1f664f4e. Report an issue: GitHub.