jestjs/jest · error · Error

Missing second argument. It must be a callback function. Per

Error message

Missing second argument. It must be a callback function. Perhaps you want to use `test.todo` for a test placeholder.

What it means

it()/test() requires a callback as the second argument. Env.ts:554-558 throws when fn is undefined, with a hint pointing to test.todo for placeholder tests. This distinguishes 'I forgot the body' from 'I want a placeholder' and nudges toward the right API.

Source

Thrown at packages/jest-jasmine2/src/jasmine/Env.ts:555

        return spec;

        function specResultCallback(result: SpecResult) {
          clearResourcesForRunnable(spec.id);
          currentSpec = null;
          reporter.specDone(result);
        }

        function specStarted(spec: Spec) {
          currentSpec = spec;
          defaultResourcesForRunnable(spec.id, suite.id);
          reporter.specStarted(spec.result);
        }
      };

      this.it = function (description, fn, timeout) {
        description = convertDescriptorToString(description);
        if (fn === undefined) {
          throw new Error(
            'Missing second argument. It must be a callback function. Perhaps you want to use `test.todo` for a test placeholder.',
          );
        }
        if (typeof fn !== 'function') {
          throw new TypeError(
            `Invalid second argument, ${fn}. It must be a callback function.`,
          );
        }
        const spec = specFactory(
          description,
          fn,
          currentDeclarationSuite,
          timeout,
        );
        if (currentDeclarationSuite.markedPending) {
          spec.pend();
        }

View on GitHub (pinned to f49721c78e)

Solutions

1. If you want a placeholder, use test.todo('does a thing') instead of it('does a thing').
2. Otherwise, add the callback: it('does a thing', () => { ... }).
3. Delete the line if it is leftover scaffolding.

Example fix

// before
it('calculates tax');

// after — intended placeholder
it.todo('calculates tax');
// or implemented
it('calculates tax', () => { expect(calc(10)).toBe(1); });

function assertItHasBody(fn: unknown) {
  if (fn === undefined) throw new Error('it needs a callback; use it.todo for placeholders');
}

function isItCall(args: [unknown, unknown]): boolean { return args.length >= 2; }

When it happens

Trigger: Calling `it('does a thing')` with no callback, or `it('does a thing', undefined)`. The check at Env.ts:554 (`fn === undefined`) throws at line 555.

Common situations: Writing a test title as a TODO before implementing the body; refactor that deleted the function but left the title; copy-paste missing the arrow function.

Related errors

• Invalid second argument, ${fn}. It must be a callback functi
• Todo must be called with only a description.
• Tests cannot be nested. Test "${spec.description}" cannot ru
• Missing second argument. It must be a callback function.
• Invalid second argument, ${specDefinitions}. It must be a ca

AI-assisted analysis of jestjs/jest@f49721c78e (2026-08-03). Data as JSON: /data/errors/9018ad59203c5793.json. Report an issue: GitHub.