node:timers/promises: make scheduler an instance of a Scheduler class like node

[robobun]

Problem

• require("timers/promises").scheduler is a plain object literal (src/js/node/timers.promises.ts, the export default block), so scheduler.constructor === Object and new scheduler.constructor() returns {}. Node throws ERR_ILLEGAL_CONSTRUCTOR.
• Because scheduler.yield is setImmediate itself, scheduler.yield("x") resolves to "x"; node's yield() takes no arguments and resolves to undefined.
• yield() / wait() run with any receiver; node throws ERR_INVALID_THIS for a receiver that is not the scheduler.
• Upstream test/parallel/test-timers-promises-scheduler.js (v26.3.0) ends with assert.throws(() => new scheduler.constructor(), { code: 'ERR_ILLEGAL_CONSTRUCTOR' }), so the vendored copy was stuck on an older revision.

Fix

• timers.promises.ts: scheduler is now the only instance of a Scheduler class with the same shape as node's lib/timers/promises.js: the constructor throws ERR_ILLEGAL_CONSTRUCTOR, yield() / wait() live on the prototype and throw ERR_INVALID_THIS("Scheduler") unless the receiver carries the module-private kScheduler brand, yield() calls setImmediate() with no arguments, wait() forwards to setTimeout(delay, undefined, options) as before. The instance is created with Object.create(Scheduler.prototype) so the throwing constructor never runs.
• internal/test_runner/mock_timers.ts: scheduler.wait mocking stored the real function bound to the MockTimers instance and restored that bound copy. With the receiver check, the restored copy throws ERR_INVALID_THIS, so mock.timers.enable() (default apis include scheduler.wait) followed by reset() would leave scheduler.wait() broken. Node 26.3.0 has exactly this bug (see details). The port now stores the own-property descriptor and restores it, or deletes the own property when there was none, which puts the inherited prototype method back.
• Why this is right: the Scheduler shape, error codes and messages match node verbatim (Illegal constructor, Value of "this" must be of type Scheduler); the mock-timers deviation is the minimum needed so that adopting node's shape does not import node's restore bug, and it is the same descriptor store/restore the rest of that file already uses for setTimeout and friends.
• setImmediate(value?, ...) is a type-only annotation so the zero-argument call in yield() typechecks; setImmediate.length is unchanged (1, as in node).
• Tests:
  • test/js/node/timers.promises/timers.promises.test.ts: new scheduler block (constructor, subclass and Reflect.construct all throw; prototype layout; ERR_INVALID_THIS for foreign receivers; real receiver and objects inheriting from it still work; yield() ignores arguments; wait() forwards delay/options). 5 of the 7 fail on the released binary, the other 2 guard the behavior that must not change.
  • test/js/node/test_runner/node-test.test.ts: mock.timers restores the prototype wait() after reset(), across repeated cycles, and restores a user-installed own wait. All 3 fail on the released binary, and all 3 also fail with only the Scheduler class change applied (first one via scheduler.wait() throwing ERR_INVALID_THIS).
  • test/js/node/test/parallel/test-timers-promises-scheduler.js refreshed to the v26.3.0 text, byte-identical to upstream now that Drop the trailing period from the node-shaped AbortError message #39277 (the AbortError message period) is on main and this branch is rebased on it. Passes with bun bd <file>; without the Scheduler change it fails at the new assert.throws.
  • Also run: test-runner-mock-timers.js, test-runner-mock-timers-date.js, test-mock-timers-abortsignal-timeout.js, test-timers-promises.js, test-timers-{timeout,immediate,interval}-promisified.js, and the full node-test.test.ts (48 pass). test-runner-mock-timers-scheduler.js passes except its 100 ms wall-clock assertion, which fails identically on unmodified main in this debug build (the file takes ~20 ms in CI per expected-durations.json).
• node:timers/promises: implement setInterval as async generator (lazy arm + iterator protocol) #34619 rewrites setInterval in the same file; this change only touches the bottom of the file and the setImmediate signature, so the two do not overlap.

Background

• ERR_ILLEGAL_CONSTRUCTOR is the node error for classes that are exported only so instanceof / .constructor work but can never be instantiated by users (performance, webcrypto, scheduler). ERR_INVALID_THIS(name) is the error node's such methods throw when called with the wrong receiver, for example scheduler.wait.call({}, 1).
• Node marks the one legitimate instance with a private symbol (kScheduler) set as an own property, and the methods check this[kScheduler]. That is why an object created with Object.create(scheduler) is accepted (it inherits the brand) while Object.create(Scheduler.prototype) is rejected; the tests cover both.
• node:test mock timers (mock.timers.enable({ apis })) replace timer functions with fakes driven by tick(), and reset() puts the originals back. For module exports they save and restore property descriptors; scheduler.wait was the one API handled by saving a bound function instead. Since the real wait is inherited, "restoring" it by assignment creates an own property, so restoring correctly means removing that own property (or putting back whatever own property existed before).

Node 26.3.0 exhibits the mock-timers restore bug this PR avoids

$ node -e '
const { mock } = require("node:test");
const { scheduler } = require("node:timers/promises");
mock.timers.enable({ apis: ["scheduler.wait"] });
mock.timers.reset();
scheduler.wait(1).then(() => console.log("ok"), e => console.log(e.code));
'

prints nothing because scheduler.wait(1) throws synchronously:

TypeError [ERR_INVALID_THIS]: Value of "this" must be of type Scheduler

lib/internal/test_runner/mock/mock_timers.js (#storeOriginalSchedulerWait / #restoreOriginalSchedulerWait) binds Scheduler.prototype.wait to the MockTimers instance, which does not carry the kScheduler brand. A build of this branch with only the timers.promises.ts change reproduced the same output; with the mock_timers.ts change it prints ok.

Shape comparison against node 26.3.0 after this change

Probing constructor name, own keys, prototype keys, yield/wait name and length, property descriptors, new, subclassing, Reflect.construct, and calls with {}, Object.create(proto), Object.create(scheduler), strings, numbers, null and undefined receivers gives identical results on node and this branch, apart from engine-specific wording of the plain TypeError thrown for a null / undefined receiver and for calling the class without new.

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• test/js/node/timers.promises/timers.promises.test.ts

Walkthrough

The timers promises scheduler now uses a singleton Scheduler instance with validated methods and timer delegation. Mock timers preserve or remove scheduler.wait according to its original property descriptor. Tests cover API compatibility and repeated mock-timer cycles.

Changes

Scheduler API and mock timer restoration

Layer / File(s)	Summary
Scheduler API and compatibility behavior src/js/node/timers.promises.ts, test/js/node/timers.promises/timers.promises.test.ts, test/js/node/test/parallel/test-timers-promises-scheduler.js	Scheduler rejects direct construction, validates receivers, and delegates yield() and wait() to the timer functions. setImmediate accepts an omitted value. Tests cover construction, inheritance, arguments, abort handling, validation, and delegation.
Mock scheduler.wait restoration src/js/internal/test_runner/mock_timers.ts, test/js/node/test_runner/node-test.test.ts	Mock timers store the original scheduler.wait property descriptor. Reset restores that descriptor or deletes the property when none existed. Tests cover inherited behavior, repeated cycles, and user-defined own implementations.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The description clearly explains the problem, implementation, rationale, and verification, although it uses different headings from the repository template.
Title check	✅ Passed	The title clearly and concisely identifies the primary change: making the timers/promises scheduler a Scheduler class instance.

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[robobun]

Status: fixed, rebased on main, waiting on CI (head 8b2ff73).

• Reproduced on bun 1.4.0 and main: new require("timers/promises").scheduler.constructor() returns {}; node 26.3.0 throws ERR_ILLEGAL_CONSTRUCTOR.
• Fail-before: USE_SYSTEM_BUN=1 bun test test/js/node/timers.promises/timers.promises.test.ts (the new scheduler tests fail) and test/js/node/test_runner/node-test.test.ts -t scheduler (3 fail); both pass with bun bd test. The vendored test-timers-promises-scheduler.js is byte-identical to upstream v26.3.0 (branch rebased on main, which has Drop the trailing period from the node-shaped AbortError message #39277) and passes with bun bd.
• The mock_timers.ts part is load-bearing: with only the timers.promises.ts change, all 3 node-test.test.ts cases fail and scheduler.wait() throws ERR_INVALID_THIS after mock.timers.reset(), which is what node 26.3.0 itself does today.
• Review follow-ups: the mock_timers comment is down to one line (78ba477). The suggestion to throw ERR_INVALID_THIS for null / undefined receivers was declined because node throws a plain TypeError there; that behavior is now pinned by a test (8b2ff73), see the thread on timers.promises.ts.
• Previous CI run (build 99065): the new tests passed on every lane; the only hard failure was test/js/bun/cron/cron.test.ts (a stale crontab entry from a sibling test, unrelated, reported separately), the rest were retry-passing flakes.

[claude]

LGTM — focused Node compat fix that mirrors lib/timers/promises.js verbatim.

What was reviewed:

• Scheduler class shape (throwing constructor, kScheduler brand check, yield() calling setImmediate() with no args) against Node's implementation; Object.create(Scheduler.prototype) correctly bypasses the throwing constructor.
• setImmediate(value?, ...) — the ? is stripped at build time so .length stays 1; $ERR_ILLEGAL_CONSTRUCTOR / $ERR_INVALID_THIS both exist in ErrorCode.ts.
• mock_timers.ts restore path now uses the same descriptor store/restore pattern as the sibling setTimeout/setInterval handlers, and correctly handles the "no own property" case by deleting to expose the prototype method.
• New tests have try/finally cleanup around mock.timers.enable() so they don't leak state into the rest of node-test.test.ts.

Extended reasoning...

Overview

This PR changes require('timers/promises').scheduler from a plain object literal to the sole instance of a Scheduler class, matching Node's lib/timers/promises.js: the constructor throws ERR_ILLEGAL_CONSTRUCTOR, yield()/wait() are prototype methods that brand-check via a module-private kScheduler symbol and throw ERR_INVALID_THIS for foreign receivers, and yield() no longer forwards arguments to setImmediate. It also fixes mock_timers.ts so mock.timers.reset() restores the inherited prototype method (via descriptor store/restore or own-property delete) rather than a bound copy that would now fail the receiver check — a bug Node itself has. Five files touched: two source files in src/js/, the vendored upstream test refreshed to v26.3.0, and two Bun test files with new coverage.

Security risks

None. Pure JS, no new inputs parsed, no privilege boundaries crossed. The brand check tightens behavior (rejects foreign receivers) rather than loosening it.

Level of scrutiny

Medium-low. This is a Node-compat shape fix in built-in JS with a reference implementation to diff against; the PR description shows the author did that comparison exhaustively. The mock_timers.ts deviation from Node is deliberate, minimal, well-commented, and reuses the exact descriptor pattern already in the file for every other mocked API. The setImmediate signature change is TypeScript-only (? strips at build), so runtime .length is unchanged.

Other factors

Test coverage is thorough: constructor/subclass/Reflect.construct all throw, prototype layout, receiver checks (accept scheduler and objects inheriting from it, reject {}/Object.create(proto)/primitives), yield() ignoring args, wait() forwarding, and three mock.timers restore scenarios including repeated cycles and a user-installed own wait. The PR description documents which tests fail on the released binary (proving they're not vacuous) and lists the wider set of related tests re-run. All mock.timers.enable() calls in the new tests are wrapped in try/finally with reset(), so they're hermetic within the shared-process test file.

[robobun]

^{Updated 12:40 AM PT - Aug 16th, 2026}

✅ @robobun, your commit 8b2ff73a7b02049a7628e4d3e0c8bd543a37d35d passed in Build #99273! 🎉

---

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Inline comments:
In `@src/js/node/timers.promises.ts`:
- Around line 242-249: Update the receiver validation in the Scheduler methods
yield and wait to use an optional brand lookup, ensuring null and undefined
receivers throw ERR_INVALID_THIS before accessing the scheduler brand. Extend
the receiver test matrix to cover scheduler.yield.call(null) and
scheduler.wait.call(undefined, 1).

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• src/js/internal/test_runner/mock_timers.ts
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• test/js/node/test/parallel/test-timers-promises-scheduler.js
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• test/js/node/timers.promises/timers.promises.test.ts

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[claude]

LGTM — a self-contained Node.js compat fix that ports the Scheduler class shape from lib/timers/promises.js verbatim, with the mock-timers restore adjusted to the same descriptor pattern the file already uses for the other timers.

What was reviewed:

• Scheduler class shape (throwing constructor, kScheduler brand check, Object.create instantiation) against node's implementation; $ERR_ILLEGAL_CONSTRUCTOR/$ERR_INVALID_THIS are existing builtin helpers.
• setImmediate(value?, ...) is type-only — .length stays 1; Object.keys(scheduler) returns [] because the brand is a symbol key.
• mock_timers store/restore: getOwnPropertyDescriptor returns undefined for the inherited method, and restore deletes the mock own property to re-expose the prototype — matches the sibling setTimeout/setInterval handling.
• Tests: receiver matrix covers branded/unbranded/nullish (the CodeRabbit thread on nullish receivers was correctly declined and pinned); mock.timers tests scope enable to scheduler.wait only and clean up in finally.

Extended reasoning...

Overview

Two built-in JS modules and three test files. src/js/node/timers.promises.ts replaces the plain-object scheduler with an instance of a Scheduler class whose constructor throws ERR_ILLEGAL_CONSTRUCTOR and whose yield()/wait() prototype methods brand-check the receiver via a module-private symbol — a direct port of node's lib/timers/promises.js. src/js/internal/test_runner/mock_timers.ts switches scheduler.wait mocking from bound-function store/restore to own-property-descriptor store/restore, so reset() re-exposes the prototype method instead of leaving a bound copy that fails the new receiver check. The vendored test-timers-promises-scheduler.js is refreshed to v26.3.0 (adds the new scheduler.constructor() assertion), and new tests in timers.promises.test.ts and node-test.test.ts cover the class shape and the mock-timers restore.

Security risks

None. Pure JS in built-in modules; no native code, no I/O, no auth/crypto, no user-controlled parsing. The brand check is a module-private Symbol(), so it is unforgeable from userland.

Level of scrutiny

Low-to-medium. This is Node compat surface with a clear reference implementation, ~30 lines of runtime code, and no new abstractions. The mock_timers change is the one deliberate divergence from upstream node (which has the restore bug the PR description reproduces), and it reuses the exact descriptor pattern the same file already applies to setTimeout/setInterval/setImmediate. $ERR_ILLEGAL_CONSTRUCTOR and $ERR_INVALID_THIS are established builtin error helpers used across src/js/.

Other factors

Test coverage is thorough by the repo's standards: exact error name/code/message assertions, subclass and Reflect.construct variants, the full receiver matrix (branded object, prototype-only object, primitives, nullish), yield() ignoring arguments, wait() option forwarding via abort, and three mock.timers restore scenarios (first cycle, repeated cycles, user-installed own wait). The author verified fail-before with USE_SYSTEM_BUN=1, ran the adjacent vendored node tests, and reported the prior CI build passing the new tests on every lane. Both prior review threads (comment-cop on the long comment, CodeRabbit on nullish receivers) were addressed — the comment was cut to one line and the nullish-receiver behavior was correctly kept node-matching and pinned by a test. No CODEOWNERS cover these paths.