jestjs/jest · error · TypeError

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

Error message

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

What it means

describe()'s second argument must be a function. Env.ts:404-408 throws a TypeError (note: TypeError, not Error) when specDefinitions is defined but not a function — e.g. an object, array, string, or config object mistakenly passed as the body. The interpolated value in the message helps spot what was actually passed.

Source

Thrown at packages/jest-jasmine2/src/jasmine/Env.ts:405

            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) {

View on GitHub (pinned to f49721c78e)

Solutions

1. Pass a function as the second argument: describe('name', () => { ... }).
2. Inspect the interpolated value in the message to identify which non-function was passed and fix the binding.
3. If wrapping describe, ensure your wrapper forwards the callback in the correct position.

Example fix

// before — object passed instead of callback
describe('calculator', { tests: true });

// after
describe('calculator', () => { it('adds', () => {}); });

function assertDescribeFn(fn: unknown) {
  if (typeof fn !== 'function') throw new TypeError('describe callback must be a function');
}

function isFn(v: unknown): v is Function { return typeof v === 'function'; }
if (!isFn(specDefinitions)) throw new TypeError('describe callback missing');

When it happens

Trigger: Calling `describe('name', someObject)`, `describe('name', [...])`, or `describe('name', 'does things')`. The `typeof specDefinitions !== 'function'` check at Env.ts:404 fires the TypeError at line 405.

Common situations: Passing a configuration object instead of a function; destructuring mistake leaving the wrong binding as the second arg; tools that wrap describe and forward the wrong parameter.

Related errors

• Missing second argument. It must be a callback function.
• describe does not expect any arguments
• Invalid second argument, ${fn}. It must be a callback functi
• Missing second argument. It must be a callback function. Per
• Tests cannot be nested. Test "${spec.description}" cannot ru

AI-assisted analysis of jestjs/jest@f49721c78e (2026-08-03). Data as JSON: /data/errors/3644f41f6ffd5f4e.json. Report an issue: GitHub.