denoland/deno · error · TypeError

beforeEach() requires a function argument

Error message

beforeEach() requires a function argument

What it means

The module-level beforeEach(fn) from node:test registers a hook run before each test in the current suite (describe block) — or on the root hooks when called outside any suite. The first line of its implementation rejects non-functions with TypeError 'beforeEach() requires a function argument'.

Source

Thrown at ext/node/polyfills/testing.ts:2192

    const tapSuite = getTapCurrentSuite();
    if (tapSuite !== null) {
      ArrayPrototypePush(tapSuite.afterAllHooks ??= [], fn);
      return;
    }
    ArrayPrototypePush(rootAfterHooks, fn);
    scheduleTapRun();
    return;
  }
  if (currentSuite) {
    ArrayPrototypePush(currentSuite.afterAllHooks, fn);
    return;
  }
  ArrayPrototypePush(rootAfterHooks, fn);
}

function beforeEach(fn, _options) {
  if (typeof fn !== "function") {
    throw new TypeError("beforeEach() requires a function argument");
  }
  if (currentSuite) {
    ArrayPrototypePush(currentSuite.beforeEachHooks, fn);
    return;
  }
  ArrayPrototypePush(rootBeforeEachHooks, fn);
}

function afterEach(fn, _options) {
  if (typeof fn !== "function") {
    throw new TypeError("afterEach() requires a function argument");
  }
  if (currentSuite) {
    ArrayPrototypePush(currentSuite.afterEachHooks, fn);
    return;
  }
  ArrayPrototypePush(rootAfterEachHooks, fn);
}

View on GitHub (pinned to 9ad36f7a2c)

Solutions

1. Pass the function reference: beforeEach(resetState)
2. Reference a specific method for helper objects: beforeEach(hooks.reset)
3. Validate dynamically sourced hooks with typeof before registering

Example fix

// before
beforeEach(seedRows());

// after
beforeEach(seedRows);

import { beforeEach } from 'node:test';

if (typeof seed !== 'function') {
  throw new TypeError(`beforeEach: expected function, got ${typeof seed}`);
}
beforeEach(seed);

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

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

When it happens

Trigger: beforeEach() with no argument; beforeEach(resetState()) passing the invocation result; beforeEach(describeBlock) passing a suite helper instead of a hook.

Common situations: Calling the helper instead of referencing it; copying hook registrations between suites and dropping the function; passing an imported object of helpers instead of a specific method.

Related errors

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

AI-assisted analysis of denoland/deno@9ad36f7a2c (2026-08-20). Data as JSON: /api/errors/76e4f36dae5a046d. Report an issue: GitHub.