denoland/deno · error · TypeError

beforeEach() requires a function

Error message

beforeEach() requires a function

What it means

t.beforeEach(fn) on the node:test context registers a hook run before each of this test's subtests. The implementation pushes fn onto an internal array and requires typeof fn === 'function'; otherwise it throws TypeError 'beforeEach() requires a function'.

Source

Thrown at ext/node/polyfills/testing.ts:1620

  }

  before(fn, _options) {
    if (typeof fn !== "function") {
      throw new TypeError("before() requires a function");
    }
    ArrayPrototypePush(this.#beforeHooks, fn);
  }

  after(fn, _options) {
    if (typeof fn !== "function") {
      throw new TypeError("after() requires a function");
    }
    ArrayPrototypePush(this.#afterHooks, fn);
  }

  beforeEach(fn, _options) {
    if (typeof fn !== "function") {
      throw new TypeError("beforeEach() requires a function");
    }
    ArrayPrototypePush(this.#beforeEachHooks, fn);
  }

  afterEach(fn, _options) {
    if (typeof fn !== "function") {
      throw new TypeError("afterEach() requires a function");
    }
    ArrayPrototypePush(this.#afterEachHooks, fn);
  }

  // Runs this context's `before()` hooks a single time, before its first
  // subtest. Idempotent so it is safe to call from every subtest's step.
  async _runBeforeHooksOnce() {
    if (this.#beforeHooksRun) return;
    this.#beforeHooksRun = true;
    // deno-lint-ignore no-this-alias
    const ctx = this;

View on GitHub (pinned to 9ad36f7a2c)

Solutions

1. Pass the function reference: t.beforeEach(seedRow)
2. Reference the specific method when importing an object of helpers: t.beforeEach(hooks.seed)
3. Check dynamically sourced hooks with typeof before registering

Example fix

// before
t.beforeEach(resetDb());

// after
t.beforeEach(resetDb);

if (typeof seed !== 'function') {
  throw new TypeError(`t.beforeEach: expected function, got ${typeof seed}`);
}
t.beforeEach(seed);

const isHookFn = (fn) => typeof fn === 'function';

if (isHookFn(seed)) {
  t.beforeEach(seed);
}

When it happens

Trigger: t.beforeEach() with no argument; t.beforeEach(seedRow(...)) passing a query result; passing a describe-style name string first.

Common situations: Seed/reset helpers invoked instead of referenced; hook helpers imported as default objects and passed wholesale (t.beforeEach(hooks) where hooks.seed was meant).

Related errors

• before() requires a function
• after() requires a function
• afterEach() requires a function
• before() requires a function argument
• after() requires a function argument

AI-assisted analysis of denoland/deno@9ad36f7a2c (2026-08-20). Data as JSON: /api/errors/9a3e5883f6eb6788. Report an issue: GitHub.