bun:test: call a function passed to .resolves/.rejects and wait on its result by robobun · Pull Request #41069 · oven-sh/bun

robobun · 2026-08-31T22:34:01Z

Problem

expect(fn).rejects and expect(fn).resolves fail with Expected promise when fn is a function. Jest calls the function and waits on what it returns: .rejects since jest 26 (feat: support async functions in toThrow jestjs/jest#9817), .resolves since jest 30.0.4 (feat: support async functions in toBe jestjs/jest#15714). Fixes Support expect(AsyncFunction).rejects #16144.
Cause: process_promise (src/runtime/test_runner/expect.rs:510) hands the receiver straight to the thenable check. A function has no then, so it fails as a non-promise.

Fix

process_promise calls a callable receiver, then waits on the return value through thenable_to_wait_for (bun:test: settle thenables through their own then() in .resolves/.rejects #40996). A synchronous throw propagates as the test error, as in Jest. The usage error takes Jest's wording: Expected a promise or a function returning a promise.
The mismatch messages print the settled value (Received promise that resolved: "x"), not the receiver, which printed as Promise { <resolved> }.
resolves in bun-types unwraps a function receiver's return type, so expect(async () => 1).resolves.toBe(1) type-checks.
Verified: test/js/bun/test/expect.test.js (new test resolves/rejects on a function returning a promise, function cases for expect.rejectsTo / expect.resolvesTo), test/integration/bun-types/fixture/test.ts. Also the other expect suites in test/js/bun/test.

Background

Stacked on bun:test: settle thenables through their own then() in .resolves/.rejects #40996, which adds thenable_to_wait_for, an adopter promise that calls a value's own then(). Merge bun:test: settle thenables through their own then() in .resolves/.rejects #40996 first. This PR is the function receiver half of bun:test: accept thenables and functions in expect .resolves/.rejects #32944.
expect.resolvesTo / expect.rejectsTo are Bun-only asymmetric matchers. They reach process_promise through Expect_readFlagsAndProcessPromise, so inside toEqual a function-valued property that faces one of them is now called.
.resolves follows Jest 30.0.4. Vitest 4.1.9 calls a function receiver for .rejects only, so the .resolves cases in expect.test.js are gated on !isVitest.

Notes

Jest semantics (packages/expect/src/index.ts, makeResolveMatcher / makeRejectMatcher): typeof actual === 'function' ? actual() : actual, then isPromise(actualWrapper) (any object or function with a callable then). The usage error prints the original receiver, not its return value. This PR keeps both.
A function receiver that returns a non-thenable (() => 4) is the usage error, with Received: [Function], as in Jest.
In the asymmetric matcher path process_promise runs with silent = true. A thrown error from the received function stays pending on the VM, Expect_readFlagsAndProcessPromise returns false, and the RETURN_IF_EXCEPTION that bun:test: settle thenables through their own then() in .resolves/.rejects #40996 adds in matchAsymmetricMatcher propagates it instead of letting .not flip the FAIL to PASS. The new expect.rejectsTo / resolvesTo propagate a sync throw from a received function test covers all four combinations.
The settled value is printed with the same console formatter the other matchers use for Received:, so an Error prints with its stack, as expect(new Error("x")).toBe(1) does today.
rejects stays Matchers<unknown>. The resolves type is Matchers<Awaited<T extends (...args: any[]) => infer R ? R : T>>. Without it, expect(() => Promise.resolve(1)).resolves.toBe(1) is TS2769: No overload matches this call. The fixture checks Matchers<number> for a promise and for a function receiver.
Fail before: on the bun:test: settle thenables through their own then() in .resolves/.rejects #40996 branch the new expect.test.js test fails with Expected promise on its first assertion, and the fixture reports four type errors without the .d.ts change.
Suites run on the debug build: expect.test.js 417 pass, bun-test.test.ts 5 pass, expect-extend.test.js, jest-extended.test.js, spyMatchers.test.ts, expect-assertions.test.ts, mock-fn.test.js, bun-types.test.ts 20 pass. BUN_JSC_validateExceptionChecks=1 on the new tests reports nothing.
Self-reviewed: 2 concerns raised. Addressed: the change was first folded into bun:test: settle thenables through their own then() in .resolves/.rejects #40996, which had excluded this feature from its scope. It is now this stacked PR, so the hang fix and the Jest parity feature are reviewed on their own. Not addressed here: the settled-promise direct read in thenable_to_wait_for skips an overridden then() on an already settled subclass. That helper belongs to bun:test: settle thenables through their own then() in .resolves/.rejects #40996, which documents the choice, and bun:test: wait on a Promise subclass through its own then() #41008 adds a shared JSPromise::awaitable that bun:test: settle thenables through their own then() in .resolves/.rejects #40996 plans to adopt.

[review] gate passed · iteration 0 · 4 files touched

fails on main (without fix)

ASAN without fix: 5 failed, 2 skipped
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test "--reporter=junit" "--reporter-outfile=/tmp/mechgate.xml" test/js/bun/test/expect.test.js
bun test v1.4.1 (a6c4cc276)

test/js/bun/test/expect.test.js:
(pass) expect() > () [300.70ms]
(pass) expect() > toBe() > expect(0).toBe(0) == true [2.20ms]
(pass) expect() > toBe() > expect(0).toBe(0) == true [0.46ms]
(pass) expect() > toBe() > expect(0).toBe(0) == true [0.24ms]
(pass) expect() > toBe() > expect(-0).toBe(-0) == true [0.25ms]
(pass) expect() > toBe() > expect(1).toBe(1) == true [0.23ms]
(pass) expect() > toBe() > expect(1).toBe(1) == true [0.23ms]
(pass) expect() > toBe() > expect(NaN).toBe(NaN) == true [0.23ms]
(pass) expect() > toBe() > expect(Infinity).toBe(Infinity) == true [0.25ms]
(pass) expect() > toBe() > expect({}).toBe({}) == true [0.23ms]
(pass) expect() > toBe() > expect(Symbol(a)).toBe(Symbol(a)) == true [0.23ms]
(pass) expect() > toBe() > expect(0).toBe(false) == false [1.25ms]
(pass) expect() > toBe() > expect(0).toBe("") == false [0.44ms]
(pass) expect() > toBe() > expect(0).toBe(-0) == false [0.34ms]
(pass) expect() > toBe() > expect(0).toBe(-0) == false [0.29ms]
(pass) e
... (truncated)

release without fix: 2 skipped
bun test v1.4.1-canary.1 (6f9e95b81)

test/js/bun/test/expect.test.js:
(pass) expect() > () [0.64ms]
(pass) expect() > toBe() > expect(0).toBe(0) == true [0.02ms]
(pass) expect() > toBe() > expect(0).toBe(0) == true
(pass) expect() > toBe() > expect(0).toBe(0) == true
(pass) expect() > toBe() > expect(-0).toBe(-0) == true
(pass) expect() > toBe() > expect(1).toBe(1) == true
(pass) expect() > toBe() > expect(1).toBe(1) == true
(pass) expect() > toBe() > expect(NaN).toBe(NaN) == true
(pass) expect() > toBe() > expect(Infinity).toBe(Infinity) == true
(pass) expect() > toBe() > expect({}).toBe({}) == true
(pass) expect() > toBe() > expect(Symbol(a)).toBe(Symbol(a)) == true
(pass) expect() > toBe() > expect(0).toBe(false) == false [0.02ms]
(pass) expect() > toBe() > expect(0).toBe("") == false
(pass) expect() > toBe() > expect(0).toBe(-0) == false
(pass) expect() > toBe() > expect(0).toBe(-0) == false
(pass) expect() > toBe() > expect(1).toBe(2) == false
(pass) expect() > toBe() > expect(1).toBe(true) == false
(pass) expect() > toBe() > expect(1).toBe("1") == false
(pass) expect() > toBe() > expect(Infinity).toBe(-Infinity) == false
(pass) expect() > toBe() > expect("foo
... (truncated)

passes on PR (with fix)

ASAN with fix: 2 skipped
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test "--reporter=junit" "--reporter-outfile=/tmp/mechgate.xml" test/js/bun/test/expect.test.js
bun test v1.4.1 (a6c4cc276)

test/js/bun/test/expect.test.js:
(pass) expect() > () [303.08ms]
(pass) expect() > toBe() > expect(0).toBe(0) == true [1.85ms]
(pass) expect() > toBe() > expect(0).toBe(0) == true [0.43ms]
(pass) expect() > toBe() > expect(0).toBe(0) == true [0.27ms]
(pass) expect() > toBe() > expect(-0).toBe(-0) == true [0.24ms]
(pass) expect() > toBe() > expect(1).toBe(1) == true [0.23ms]
(pass) expect() > toBe() > expect(1).toBe(1) == true [0.26ms]
(pass) expect() > toBe() > expect(NaN).toBe(NaN) == true [0.23ms]
(pass) expect() > toBe() > expect(Infinity).toBe(Infinity) == true [0.23ms]
(pass) expect() > toBe() > expect({}).toBe({}) == true [0.26ms]
(pass) expect() > toBe() > expect(Symbol(a)).toBe(Symbol(a)) == true [0.24ms]
(pass) expect() > toBe() > expect(0).toBe(false) == false [1.22ms]
(pass) expect() > toBe() > expect(0).toBe("") == false [0.48ms]
(pass) expect() > toBe() > expect(0).toBe(-0) == false [0.31ms]
(pass) expect() > toBe() > expect(0).toBe(-0) == false [0.30ms]
(pass) e
... (truncated)

release with fix: 2 skipped
$ bun scripts/build.ts --profile=release
[configured] bun-profile → bun (stripped) in 597ms (unchanged)
ninja: Entering directory `/workspace/bun/build/release'
[1/6] gen generated_host_exports.rs
generated_host_exports.rs: 122 exports (host=5, lazy=10, generic=107, rust=0); 244 extern-C blocks audited
[1/6] cargo bun_runtime → libbun_runtime.a
�[1m�[92m   Compiling�[0m bun_core v0.0.0 (/workspace/bun/src/bun_core)
�[1m�[92m   Compiling�[0m bun_errno v0.0.0 (/workspace/bun/src/errno)
�[1m�[92m   Compiling�[0m bun_ptr v0.0.0 (/workspace/bun/src/ptr)
�[1m�[92m   Compiling�[0m bun_boringssl_sys v0.0.0 (/workspace/bun/src/boringssl_sys)
�[1m�[92m   Compiling�[0m bun_safety v0.0.0 (/workspace/bun/src/safety)
�[1m�[92m   Compiling�[0m bun_base64 v0.0.0 (/workspace/bun/src/base64)
�[1m�[92m   Compiling�[0m bun_cares_sys v0.0.0 (/workspace/bun/src/cares_sys)
�[1m�[92m   Compiling�[0m bun_zlib_sys v0.0.0 (/workspace/bun/src/zlib_sys)
�[1m�[92m   Compiling�[0m bun_zstd v0.0.0 (/workspace/bun/src/zstd)
�[1m�[92m   Compiling�[0m bun_picohttp v0.0.0 (/workspace/bun/src/picohttp)
�[1m�[92m   Compiling�[0m bun_brotli v0.0.0 (/workspace/bun/src/brotli)
�[1m�[92m   Compiling�[0m
... (truncated)

diff hotspot

packages/bun-types/test.d.ts               | 10 ++--
 src/runtime/test_runner/expect.rs          | 14 +++--
 test/integration/bun-types/fixture/test.ts | 19 +++++++
 test/js/bun/test/expect.test.js            | 83 +++++++++++++++++++++++++++++-
 4 files changed, 118 insertions(+), 8 deletions(-)

gate history · 1 passed · 0 rejected · iteration 0

evidence per changed file

file                                        reads  edits  tests
packages/bun-types/test.d.ts                    1      1      0
src/runtime/test_runner/expect.rs               2      4      0
test/integration/bun-types/fixture/test.ts      1      3      0
test/js/bun/test/expect.test.js                 4      5      0

…ects expect(value).resolves and .rejects waited on the internal JSPromise state and never called the value's then(). A Promise subclass that starts its work inside an overridden then(), such as Bun.SQL's Query or Bun.$'s ShellPromise, never settled, so the matcher spun the event loop forever and the test timeout could not interrupt it. A plain thenable was refused with "Expected promise". Adopt a pending promise or a thenable through a fresh native promise, which runs the spec resolve steps and calls the value's own then(), the same as await does. JSC skips that call for a promise with the built-in then. A settled promise is still read directly. The same path covers expect(fn).toThrow() on a returned promise and a custom matcher that returns a promise.

…child fixture

Let the adopter's resolve steps do the single Get(then), as await does. A value that is not a thenable fulfills the adopter at once, which is the signal for the Expected promise error. A then getter that throws now rejects instead of throwing from the matcher, the same as await. The hang guard test also asserts that the child exited on its own.

…s result Jest calls a function receiver and waits on what it returns. `.rejects` has done this since jest 26 (jestjs/jest#9817), `.resolves` since jest 30.0.4 (jestjs/jest#15714). The usage error now uses Jest's wording, "Expected a promise or a function returning a promise", and the settlement mismatch messages print the settled value instead of the receiver, which is not a promise when it is a thenable or a function. The `resolves` type unwraps a function receiver's return type, so `expect(async () => 1).resolves.toBe(1)` type-checks. The function receiver handling comes from #32944. Fixes #16144

… value CI runs the matchers with colors on, so the message reads `Received promise that resolved: \x1b[31m"settled to this"\x1b[0m`.

claude

Code review found no issues

No high-confidence issues detected in this change.

robobun · 2026-08-31T23:01:31Z

Pushed 104e0d5, a test-only change. CI runs the matchers with colors on, so the mismatch message reads Received promise that resolved: <red>"settled to this"<reset>. The three new assertions on that message now allow a color code between the label and the value. The first CI run failed only on those assertions, on every lane. FORCE_COLOR=1 bun bd test test/js/bun/test/expect.test.js reproduces the failure with the old pattern and passes with the new one.

robobun · 2026-08-31T23:06:24Z

^{Updated 4:06 PM PT - Aug 31st, 2026}

✅ @robobun, your commit 104e0d548de019c73203c0ec43c598a168055e85 passed in Build #108924! 🎉

🧪 To try this PR locally:

bunx bun-pr 41069

That installs a local version of the PR into your bun-41069 executable, so you can run:

bun-41069 --bun

robobun · 2026-11-01T09:08:47Z

Main already has the change in this PR's title. #44833 (9e76acc) added it, and #16144 is closed.

Main calls a function passed to .resolves or .rejects and awaits what it returns (received_promise, expect.rs:529). bun test: Vitest compatibility #44833 added that function and a test for it (expect-deferred.test.ts:891).
I ran the two cases from Support expect(AsyncFunction).rejects #16144 (awaited) on a debug build of main at 43fc90a. Both pass. Both fail with Expected promise on a build from before the merge (367d939).
Main also has the resolves type for a function receiver (test.d.ts:1961).

This branch cannot land as it is. It has conflicts with main in four files. It also builds on the blocking wait in the test runner, which #44833 removed. Its base PR, #40996, is closed.

Three parts of this PR are not on main. Each of them can be proposed again on top of main's code.

The mismatch message with the settled value. This PR prints Received promise that resolved: "x". Main prints the received value: Promise { <resolved> } for a promise (pinned here), and [Function] for a function.
Jest's wording for the usage error. This PR prints Expected a promise or a function returning a promise. Main prints Expected promise (pinned here).
A function under expect.resolvesTo / expect.rejectsTo. On main, expect({ a: () => Promise.resolve("x") }).toEqual({ a: expect.resolvesTo.stringContaining("x") }) fails. That path does not call a function (expect.rs:660).

robobun added 7 commits August 31, 2026 02:39

test: pin matcher failure messages and dispose the SQL client in the …

09e3fa0

…child fixture

Shorten the thenable adoption comments

a88ad9c

Shorten the thenable_to_wait_for doc comment

30db63e

Trim the thenable_to_wait_for doc comment to two lines

78690b9

robobun requested a review from alii as a code owner August 31, 2026 22:34

github-actions Bot added the claude label Aug 31, 2026

This was referenced Aug 31, 2026

bun:test: accept thenables and functions in expect .resolves/.rejects #32944

Closed

bun:test: settle thenables through their own then() in .resolves/.rejects #40996

Closed

test: let a color code sit between the mismatch label and the settled…

104e0d5

… value CI runs the matchers with colors on, so the message reads `Received promise that resolved: \x1b[31m"settled to this"\x1b[0m`.

claude Bot reviewed Aug 31, 2026

View reviewed changes

robobun force-pushed the robobun/97c4d3a2/expect-await-thenable branch from c6a9b4b to 0a65c0b Compare September 23, 2026 13:21

robobun mentioned this pull request Oct 6, 2026

bun:test: .resolves.toThrow() counts a fulfilled value as thrown only when it is an error #44652

Open

robobun closed this Nov 1, 2026

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bun:test: call a function passed to .resolves/.rejects and wait on its result - #41069

bun:test: call a function passed to .resolves/.rejects and wait on its result#41069
robobun wants to merge 8 commits into
robobun/97c4d3a2/expect-await-thenablefrom
robobun/a5b8c3a4/expect-fn-receiver

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