denoland/deno · error · TypeError

ERR_INVALID_ARG_TYPE

ERR_INVALID_ARG_TYPE

Error message

The "promiseFn" argument must be an instance of Function or Promise. Received ${promiseFn}

What it means

Thrown by assert.rejects()/assert.doesNotReject() when the first argument is neither a function nor a Promise. waitForActual() accepts only those two shapes; anything else (number, string, plain object without a thenable interface, undefined) is rejected immediately with ERR_INVALID_ARG_TYPE.

Source

Thrown at ext/node/polyfills/assert.ts:362

async function waitForActual(
  promiseFn,
) {
  let resultPromise;
  if (typeof promiseFn === "function") {
    // Return a rejected promise if `promiseFn` throws synchronously.
    resultPromise = promiseFn();
    // Fail in case no promise is returned.
    if (!checkIsPromise(resultPromise)) {
      throw new ERR_INVALID_RETURN_VALUE(
        "instance of Promise",
        "promiseFn",
        resultPromise,
      );
    }
  } else if (checkIsPromise(promiseFn)) {
    resultPromise = promiseFn;
  } else {
    throw new ERR_INVALID_ARG_TYPE(
      "promiseFn",
      ["Function", "Promise"],
      promiseFn,
    );
  }

  try {
    await resultPromise;
  } catch (e) {
    return e;
  }
  return NO_EXCEPTION_SENTINEL;
}

function expectsError(
  stackStartFn,
  actual,
  error,

View on GitHub (pinned to 89f33cbef2)

Solutions

1. Pass a Promise: assert.rejects(promise)
2. Pass a function that returns the Promise: assert.rejects(() => promise)
3. If the variable may be undefined, check it before the call and fail early

Example fix

// before
assert.rejects(maybePromise);
// after
assert.rejects(Promise.resolve(maybePromise));

if (!isFnOrPromise(target)) throw new TypeError('assert.rejects expects a function or a Promise');
await assert.rejects(target, /boom/);

const isFnOrPromise = (v) => typeof v === 'function' || (v != null && typeof v.then === 'function');

When it happens

Trigger: assert.rejects(123); assert.doesNotReject('ready'); assert.rejects({ then: 'not a function' }); passing a variable that is undefined at runtime.

Common situations: Passing a computed value or flag instead of the operation to await; a variable expected to hold a promise but undefined after a refactor; copy-pasting assert(value) style calls into assert.rejects().

Related errors

• ERR_INVALID_ARG_VALUE
• ERR_INVALID_RETURN_VALUE
• ERR_CONSTRUCT_CALL_REQUIRED
• ERR_ASSERTION
• ERR_AMBIGUOUS_ARGUMENT

AI-assisted analysis of denoland/deno@89f33cbef2 (2026-08-16). Data as JSON: /api/errors/92740ebe5e02cb5a. Report an issue: GitHub.