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To test partial failures with Promise.allSettled(), make the asynchronous operations settle in a controlled way, await the aggregate result, and assert each result by its input position. A rejected operation should appear as a { status: "rejected", reason } record; it should not reject the aggregate promise. The aggregate fulfills only after every input settles, including an empty input iterable. MDN documents the result format and behavior.
What a Promise.allSettled() test should prove
A useful test checks the contract your code relies on, not merely that the built-in method exists. Promise.allSettled() takes an iterable of inputs and fulfills with an array of outcome records after all inputs settle. Each record is either { status: "fulfilled", value } or { status: "rejected", reason }. Results stay in the same order as the inputs, even if the operations finish in another order. The ECMAScript 2025 specification defines the aggregation behavior.
- Assert the result length and the status of each relevant slot.
- For a fulfilled slot, assert its
value. - For a rejected slot, assert its
reason, using the error type or identity your application promises to expose. - When timing matters, control settlement explicitly rather than relying on delays.
Test a mixed success and failure
Here is a runner-agnostic example using Node.js’s built-in test runner and assertions. The function under test accepts two operations and aggregates their outcomes; the test controls each operation with a deferred promise.
import test from 'node:test';
import assert from 'node:assert/strict';
function deferred() {
let resolve;
let reject;
const promise = new Promise((res, rej) => {
resolve = res;
reject = rej;
});
return { promise, resolve, reject };
}
function collectOutcomes(loadProfile, loadSettings) {
return Promise.allSettled([loadProfile(), loadSettings()]);
}
test('reports a fulfillment and a rejection in input order', async () => {
const profile = deferred();
const settings = deferred();
const failure = new Error('settings unavailable');
const aggregate = collectOutcomes(
() => profile.promise,
() => settings.promise,
);
// Settle in the opposite order from the input list.
settings.reject(failure);
profile.resolve({ id: 42 });
const results = await aggregate;
assert.equal(results.length, 2);
assert.deepEqual(results[0], {
status: 'fulfilled',
value: { id: 42 },
});
assert.equal(results[1].status, 'rejected');
assert.equal(results[1].reason, failure);
});The rejection assertion checks both the status and the promised error object. If your public contract guarantees only an error message or class, assert that instead of object identity. Keep these application-level assertions separate from assumptions about the native combinator: the test should verify that your wrapper supplies the intended inputs and exposes the resulting records as expected.
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Prove the aggregate waits for every input
A mixed-result test alone may not make the wait guarantee obvious. To verify it, leave one input pending after settling the others, then check that the aggregate has not completed. Settle the final input and inspect the full result array.
test('does not finish until every input settles', async () => {
const first = deferred();
const second = deferred();
const final = deferred();
const aggregate = Promise.allSettled([
first.promise,
second.promise,
final.promise,
]);
let completed = false;
const observed = aggregate.then((results) => {
completed = true;
return results;
});
first.resolve('ready');
second.reject(new Error('unavailable'));
await Promise.resolve();
assert.equal(completed, false);
final.resolve('finished');
const results = await observed;
assert.equal(completed, true);
assert.deepEqual(results.map((result) => result.status), [
'fulfilled',
'rejected',
'fulfilled',
]);
});The microtask turn lets already-queued promise reactions run before the pending-state assertion; it does not use an arbitrary wall-clock timeout. Explicit controls make the test independent of machine speed and scheduling luck. This test exercises the native aggregate directly. For a wrapper, call the wrapper and make sure the final input remains pending through the wrapper’s own execution path.
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Cover input shapes and construction errors
Empty input
An empty iterable produces an aggregate that fulfills with an empty array. This can matter when your wrapper builds its list dynamically:
const results = await Promise.allSettled([]);
assert.deepEqual(results, []);MDN’s examples include the empty iterable case.
A plain value among promises
Inputs do not all need to be promises. A plain value is represented as a fulfilled result, which is useful if the input collection can mix cached values and asynchronous work:
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const results = await Promise.allSettled([
Promise.resolve('remote'),
'cached',
]);
assert.deepEqual(results, [
{ status: 'fulfilled', value: 'remote' },
{ status: 'fulfilled', value: 'cached' },
]);Synchronous errors while creating inputs
A function call made while building the input array can throw before Promise.allSettled() is invoked. That is a different failure path from a promise that rejects after being returned. Test it at the wrapper boundary when relevant:
function collectOutcomes(makeFirst, makeSecond) {
return Promise.allSettled([makeFirst(), makeSecond()]);
}
assert.throws(
() => collectOutcomes(
() => { throw new Error('could not construct operation'); },
() => Promise.resolve('ok'),
),
/could not construct operation/,
);Because array elements are evaluated before the call, this synchronous exception prevents the aggregate from being created. It should not be asserted as a rejected result record.
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Choose the test around the caller’s failure policy
Use the combinator that matches what the caller needs to learn from the operations:
| Method | Behavior on an input rejection | Use when |
|---|---|---|
Promise.all() |
The aggregate rejects when an input rejects. | Every operation must succeed for the combined operation to count as successful. |
Promise.allSettled() |
The aggregate fulfills after all inputs settle, returning a record for each outcome. | Operations are independent and the caller needs a complete report, including partial failures. |
MDN’s Promise.all() reference describes its rejection behavior. A test that asserts a rejected record is specifically testing the all-settled policy; it should not silently treat that outcome as equivalent to the fail-fast policy.
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Use your project’s test runner and mock at the boundary
The controlled-promise pattern works with any JavaScript test framework: create deferred inputs, invoke the code under test, settle the inputs in the required sequence, and assert the result. In Node.js, the v26.10.0 test-runner documentation describes asynchronous tests and mocking facilities. Its module-mocking facility has startup-flag and loader caveats, so check the documentation for the Node.js version you actually run before depending on it.
When testing a wrapper that calls a network or storage dependency, mock that external boundary so the test controls success, rejection, and pending behavior. Do not mock Promise.allSettled() in a test intended to verify the wrapper’s aggregation behavior; doing so would replace the behavior the test is supposed to exercise.
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