jestjs/jest · error · TypeError

Invalid first argument. It must be a callback function.

Error message

Invalid first argument. It must be a callback function.

What it means

The global `beforeEach` (and the identically-guarded afterEach, beforeAll, afterAll) wrappers in the jasmine interface require their first argument to be a function. If a non-function is passed (string, number, undefined), the wrapper throws a TypeError at jasmineLight.ts:120 before delegating to env.beforeEach. This catches mistakes where the hook is called with no body or wrong argument.

Solutions

1. Pass a function: `beforeEach(() => { ... })`.
2. If referencing a named function, pass the reference without invoking: `beforeEach(setup)` not `beforeEach(setup())`.
3. For async hooks, use `beforeEach(async () => { ... })` or `beforeEach((done) => { ... })`.

Example fix

// before
beforeEach;  // missing call/callback
// or
beforeEach(setup());
// after
beforeEach(() => { /* ... */ });
// or
beforeEach(setup);

// Guard all hook registrations.
function safeBeforeEach(fn: unknown) {
  if (typeof fn !== 'function') {
    throw new TypeError(`beforeEach requires a callback function, got ${typeof fn}`);
  }
  return beforeEach(fn as () => void);
}

function isHookFn(value: unknown): value is () => void | Promise<void> {
  return typeof value === 'function';
}

When it happens

Trigger: Calling `beforeEach()` with no arguments, `beforeEach('setup')`, or `beforeEach(undefined)`. The check `typeof arguments[0] !== 'function'` at jasmineLight.ts:119 triggers the throw. The same pattern guards afterEach (127), beforeAll (137), and afterAll (147).

Common situations: Writing `beforeEach` without a callback; passing a config object where the function was expected; a refactor that extracted the hook body into a variable that is undefined; calling `beforeEach(someFn())` instead of `beforeEach(someFn)`.

Related errors

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

AI-assisted analysis of jestjs/jest@8e6d128e4a (2026-08-10). Data as JSON: /api/errors/9f6f0933835b274f. Report an issue: GitHub.

Appendix: source

Thrown at packages/jest-jasmine2/src/jasmine/jasmineLight.ts:120

    fdescribe(description: string, specDefinitions: SpecDefinitionsFn) {
      return env.fdescribe(description, specDefinitions);
    },

    it() {
      return env.it.apply(env, arguments);
    },

    xit() {
      return env.xit.apply(env, arguments);
    },

    fit() {
      return env.fit.apply(env, arguments);
    },

    beforeEach() {
      if (typeof arguments[0] !== 'function') {
        throw new TypeError(
          'Invalid first argument. It must be a callback function.',
        );
      }
      return env.beforeEach.apply(env, arguments);
    },

    afterEach() {
      if (typeof arguments[0] !== 'function') {
        throw new TypeError(
          'Invalid first argument. It must be a callback function.',
        );
      }
      return env.afterEach.apply(env, arguments);
    },

    beforeAll() {
      if (typeof arguments[0] !== 'function') {
        throw new TypeError(

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