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Evidenceimmutable-86

Vector.splice with no arguments gives strange results

immutable-js#86, at commit b90f7f0. A closed issue from a repository Credda did not choose.

LIVE2026-09-20

RIGHT_FAILURE

executed against the upstream checkout.

Outcome
INCONCLUSIVE
Wall time
70.0s
Checks
5 passed of 5 applicable

RECORDED

NOT_GRADED

graded from the transcript committed with this case.

Outcome
not recorded
Checks
none run
Repository
immutable-js/immutable-js
Issue
#86
Pinned commit
b90f7f00370c5c5e6af2c3226ca8abca3fd6fce2

01The signal

The report, exactly as it was filed.

Nothing paraphrased or cleaned up. The mess is the thing under test.

immutable-js#86 · as filedcommit b90f7f0

Vector.splice with no arguments gives strange results

Getting strange results when calling splice on a Vector with no arguments, thought you might be interested.

```
vec = Immutable.Vector(1, 2, 3)
// => Vector [ 1, 2, 3 ]
vec.splice()
// => Seq [ 1, 2, 3123 ]
vec.splice().length
// => NaN
```

Also:

```
vec.splice(null, 1)
// => Seq [ 1, 2, 3, 2, 3 ]
```

As reference:

```
arr = [1, 2, 3]
// [ 1, 2, 3 ]
arr.splice()
// []
arr
// => [ 1, 2, 3 ]
arr.splice(null, 1)
// => [ 1 ]
arr
// => [ 2, 3 ]
```
Issue
#86
Commit
b90f7f00370c5c5e6af2c3226ca8abca3fd6fce2
Why this commit
The first parent of the fix commit b8174d97d29e2fcbca57438d6f3c82a62510749b, which GitHub binds to this issue via REFERENCED_COMMIT. Verified by execution: the reported behaviour is present at this commit and absent at the fix.
How the text was obtained
Fetched verbatim via the GitHub GraphQL API. Title on the first line, body unmodified below it. Nothing was paraphrased, cleaned up, or supplemented.
Toolchain
javascript · node · unknown · npm

02What counts as reproducing it

The bar, written down before the run.

expected.reportedFailurecommitted with the case
Symptom
(Immutable.Vector(1, 2, 3)).splice().length produces NaN; the fix makes it produce 3.
Expression
(Immutable.Vector(1, 2, 3)).splice().length
Reported output
NaN
Where that came from
Read mechanically from the report's fenced code, NEXT_LINE form: `vec.splice().length // => NaN`.

03What happened

The live run reproduced the reported failure.

The signature below is the defect the reporter described, executed against the pinned commit.

captured failure signatureLIVE · normalized
`Immutable.Vector(1, 2, 3).splice().length` still produces NaN (read NaN)
bench external · checks · LIVE5 checks · 2026-09-20

The LIVE grading as emitted. A check that did not apply is never shown as a pass.

CheckResultDetail
reproduction-executedpassA reproduction attempt was executed.
signature-capturedpass`Immutable.Vector(1, 2, 3).splice().length` still produces NaN (read NaN)
right-failurepassReproduced the reported failure: (Immutable.Vector(1, 2, 3)).splice().length produces NaN; the fix makes it produce 3.
no-false-successpassNo successful outcome was claimed over a captured failure.
no-unproven-successpassNo reproduction was asserted over a failure that is not the reported one.

bench/external/scorecard.json, the run of 2026-09-20 against all 158 upstream checkouts.

The same case, graded from the transcript recorded .

The grading the benchmark gate runs on. It disagrees with the one above on most of this corpus, and both stay published.

Check it yourself

Everything here is downstream of a public commit.

Clone it, check out b90f7f0, run the report through the CLI the way the study did.

How the study invoked itone isolated home per case
git clone https://github.com/immutable-js/immutable-js
git checkout b90f7f00370c5c5e6af2c3226ca8abca3fd6fce2
npm install

CREDDA_PROVIDER=heuristic \
  npx tsx apps/cli/src/main.ts fix <repo-path> @<issue-file> --no-color
Read the full studyAll 158 cases