jestjs/jest · error · TypeError

Invalid second argument

Error message

Invalid second argument, ${specDefinitions}. It must be a callback function.

What it means

describe's second argument must be a function. If a non-undefined, non-function value is passed (a string, object, number, array), Env.describe throws a TypeError naming the offending value. This is a stricter check than the undefined guard and catches wrong-typed arguments.

Solutions

1. Ensure the second argument is a function: `describe('x', () => {})`.
2. If you intended a factory/helper, call it so it returns a function, or wrap: `describe('x', () => myHelper())`.
3. Double-check the value of any variable passed — log it before the describe call.

Example fix

// before
describe('my suite', { timeout: 5000 });
// after
describe('my suite', () => {
  beforeEach(() => {}, 5000);
});

function safeDescribe(name: string, fn: unknown) {
  if (typeof fn !== 'function') {
    throw new TypeError(`describe('${name}') second argument must be a function, got ${typeof fn}`);
  }
  return describe(name, fn as () => void);
}

function isFunction(value: unknown): value is Function {
  return typeof value === 'function';
}

When it happens

Trigger: Calling `describe('x', 'not a function')` or `describe('x', { foo: 1 })` or passing a config object where the callback was expected. The guard at Env.ts:405 checks `typeof specDefinitions !== 'function'`.

Common situations: Calling describe with an options object instead of a callback; passing the result of an assignment expression that returned a non-function; a helper that returns the wrong type; copy-paste from a table-driven test where the table was passed as the second arg.

Related errors

• describe does not expect any arguments
• Invalid first argument. It must be a callback function.
• Invalid second argument
• Missing second argument. It must be a callback function.
• Argument passed to callFake should be a function, got

AI-assisted analysis of jestjs/jest@8e6d128e4a (2026-08-10). Data as JSON: /api/errors/3644f41f6ffd5f4e. Report an issue: GitHub.

Appendix: source

Thrown at packages/jest-jasmine2/src/jasmine/Env.ts:406

            return j$.testPath;
          },
        });

        return suite;
      };

      this.describe = function (
        description: Circus.TestNameLike,
        specDefinitions,
      ) {
        const suite = suiteFactory(description);
        if (specDefinitions === undefined) {
          throw new Error(
            'Missing second argument. It must be a callback function.',
          );
        }
        if (typeof specDefinitions !== 'function') {
          throw new TypeError(
            `Invalid second argument, ${specDefinitions}. It must be a callback function.`,
          );
        }
        if (specDefinitions.length > 0) {
          throw new Error('describe does not expect any arguments');
        }
        if (currentDeclarationSuite.markedPending) {
          suite.pend();
        }
        if (currentDeclarationSuite.markedTodo) {
          // @ts-expect-error TODO Possible error: Suite does not have todo method
          suite.todo();
        }
        addSpecsToSuite(suite, specDefinitions);
        return suite;
      };

      this.xdescribe = function (description, specDefinitions) {

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