node:events: fix ReferenceError in EventEmitter.prototype.listenerCount

[robobun]

Repro

const { EventEmitter } = require("events");
new EventEmitter().listenerCount("x");

ReferenceError: listenerCountSlow is not defined
      at listenerCount (node:events:450:29)

Every single-argument emitter.listenerCount(type) call hits this, which broke test/js/node/http2/node-http2.test.js (38 fail) via emitSessionCloseNT calling listenerCount("close"), along with anything else that counts listeners.

Cause

#34605 (efab3b2) removed the listenerCountSlow helper when it dropped the fallback path from the static events.listenerCount, but EventEmitterPrototype.listenerCount (added in a227ad9) still called it for the common 1-argument form.

Fix

Inline the lookup into the prototype method, structured the same way as Node's own listenerCount: read _events[type] once, then handle the bare-function and array storage shapes for both the 1-arg (return 1 / handlers.length) and 2-arg (match against method) forms.

Verification

• bun bd test test/js/node/events/event-emitter.test.ts: 70 pass
• New EventEmitter.prototype.listenerCount test fails on main with ReferenceError: listenerCountSlow is not defined, passes with the fix
• All 26 test/js/node/test/parallel/test-event-emitter-*.js pass
• Test expectations checked against Node.js

[robobun]

^{Updated 6:05 PM PT - Jul 18th, 2026}

@robobun, your commit 3a53a9a is building: #75562

[robobun]

Reproduced with bun bd -e 'new (require("events").EventEmitter)().listenerCount("x")' on main (dd88b5c): ReferenceError: listenerCountSlow is not defined. Fix applied; 70 tests pass in event-emitter.test.ts, all 26 test-event-emitter-*.js parallel tests pass.

[coderabbitai]

Walkthrough

EventEmitterPrototype.listenerCount now directly counts stored handlers for absent, single-function, and array-based listener representations. Tests cover specific listener matching, multiple registrations, once listeners, and null/undefined count-all behavior.

Changes

Listener count behavior

Layer / File(s)	Summary
Listener count implementation and validation src/js/node/events.ts, test/js/node/events/event-emitter.test.ts	listenerCount directly evaluates stored handlers and tests counts for missing listeners, matching listeners, multiple registrations, once listeners, and omitted or null listener arguments.

Possibly related PRs

• oven-sh/bun#34519: Updates the same listenerCount implementation and related tests.

Suggested reviewers: jarred-sumner

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and accurately summarizes the main fix: preventing a ReferenceError in EventEmitter.prototype.listenerCount.
Description check	✅ Passed	The description covers what changed and how it was verified, even though it uses custom headings instead of the template labels.
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.

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@test/js/node/events/event-emitter.test.ts`:
- Around line 919-944: Extend the EventEmitter.prototype.listenerCount test
immediately after the first ee.on("x", a) call with null and undefined
listener-argument assertions expecting a count of 1, covering count-all behavior
when a single function is stored. Keep the existing array-storage assertions
unchanged.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

Plan: Pro

Run ID: b87db90b-121d-4cda-8cf7-00483452ed18

📥 Commits

Reviewing files that changed from the base of the PR and between dd88b5c and 3a53a9a.

📒 Files selected for processing (2)

• src/js/node/events.ts
• test/js/node/events/event-emitter.test.ts

[claude]

LGTM — straightforward regression fix for a dangling reference to a deleted helper.

What was reviewed:

• New listenerCount body matches Node's reference implementation structure (bare-function vs array storage, method != null gating both branches).
• handlers === undefined check covers the shape-mode case where removed listeners are stored as undefined rather than deleted.
• Test covers all storage shapes (empty / single function / array), the .listener unwrap for once() wrappers, and null/undefined method arg — verified expectations against Node.

Extended reasoning...

Overview

EventEmitterPrototype.listenerCount was calling listenerCountSlow, a helper deleted in #34605, so every 1-arg emitter.listenerCount(type) call threw a ReferenceError on main. This PR inlines the lookup: read this._events?.[type] once, then branch on function-vs-array storage shape for both the 1-arg (count all) and 2-arg (count matching method) forms. A new test in event-emitter.test.ts covers all combinations.

Security risks

None. Pure JS, no untrusted input parsing, no privilege boundaries.

Level of scrutiny

Low-to-medium. This is a hot-path built-in module, but the change is a small, mechanical inline of previously-working logic that mirrors Node's own listenerCount almost line-for-line. The old code's falsy check (!handlers) became a strict === undefined check, which matches Node and correctly handles the kShapeMode path where cleared slots hold undefined. Handlers that aren't undefined or a function are always arrays (per _addListener), so handlers.length is safe.

Other factors

The regression was already breaking 38 tests in node-http2.test.js on main, so this is a build-fixer. The new test would have failed on main with the exact ReferenceError, and the PR reports all 26 test-event-emitter-*.js parallel tests pass. No prior reviewer comments to address.