events: optimize once() and removeListener() by mcollina · Pull Request #64373 · nodejs/node

mcollina · 2026-07-08T21:09:43Z

Optimize the EventEmitter hot paths:

rewrite the once() wrapper as a closure instead of bind() + a state object
keep the _events object shape in removeListener(): store undefined instead of delete-ing keys or reallocating the object (eventNames() now filters, matching Stream.prototype.eventNames); removes the now-unused kShapeMode
outline the unhandled error path from emit() (439 → 278 bytecodes) and the max-listeners warning from _addListener() (405 → 288) so both fit within the V8 inlining budgets

benchmark/compare.js --runs 30 before/after all changes:

events

                                                                 confidence improvement accuracy (*)   (**)  (***)
events/ee-add-remove.js n=1000000 removeListener=0 newListener=0        ***     12.59 %       ±1.59% ±2.11% ±2.76%
events/ee-add-remove.js n=1000000 removeListener=0 newListener=1          *      2.37 %       ±1.92% ±2.58% ±3.40%
events/ee-add-remove.js n=1000000 removeListener=1 newListener=0        ***     15.12 %       ±1.14% ±1.52% ±1.99%
events/ee-add-remove.js n=1000000 removeListener=1 newListener=1        ***     21.82 %       ±2.24% ±3.01% ±3.97%
events/ee-emit.js listeners=1 argc=0 n=2000000                                   0.89 %       ±3.72% ±4.95% ±6.47%
events/ee-emit.js listeners=1 argc=10 n=2000000                                 -0.85 %       ±2.46% ±3.28% ±4.28%
events/ee-emit.js listeners=1 argc=2 n=2000000                                   1.25 %       ±3.73% ±4.97% ±6.50%
events/ee-emit.js listeners=1 argc=4 n=2000000                                  -0.11 %       ±2.91% ±3.87% ±5.04%
events/ee-emit.js listeners=10 argc=0 n=2000000                                  0.99 %       ±1.95% ±2.60% ±3.38%
events/ee-emit.js listeners=10 argc=10 n=2000000                                -0.24 %       ±1.64% ±2.18% ±2.84%
events/ee-emit.js listeners=10 argc=2 n=2000000                                  0.05 %       ±1.61% ±2.14% ±2.79%
events/ee-emit.js listeners=10 argc=4 n=2000000                          **      2.22 %       ±1.60% ±2.13% ±2.77%
events/ee-emit.js listeners=5 argc=0 n=2000000                                   1.82 %       ±2.04% ±2.72% ±3.54%
events/ee-emit.js listeners=5 argc=10 n=2000000                                 -0.26 %       ±1.61% ±2.14% ±2.79%
events/ee-emit.js listeners=5 argc=2 n=2000000                            *     -2.25 %       ±1.96% ±2.61% ±3.39%
events/ee-emit.js listeners=5 argc=4 n=2000000                                   0.64 %       ±2.11% ±2.81% ±3.66%
events/ee-listen-unique.js n=1000000 events=1                           ***     14.85 %       ±0.80% ±1.06% ±1.39%
events/ee-listen-unique.js n=1000000 events=10                          ***      8.46 %       ±1.28% ±1.71% ±2.23%
events/ee-listen-unique.js n=1000000 events=2                           ***     14.09 %       ±0.88% ±1.18% ±1.54%
events/ee-listen-unique.js n=1000000 events=20                          ***      7.53 %       ±1.38% ±1.84% ±2.40%
events/ee-listen-unique.js n=1000000 events=3                           ***      9.24 %       ±1.83% ±2.44% ±3.20%
events/ee-listen-unique.js n=1000000 events=5                           ***     13.22 %       ±1.39% ±1.86% ±2.45%
events/ee-listener-count-on-prototype.js n=50000000                              0.14 %       ±0.82% ±1.09% ±1.41%
events/ee-listeners.js raw='false' listeners=5 n=5000000                ***     -3.03 %       ±1.73% ±2.31% ±3.03%
events/ee-listeners.js raw='false' listeners=50 n=5000000                       -0.00 %       ±0.54% ±0.72% ±0.94%
events/ee-listeners.js raw='true' listeners=5 n=5000000                 ***      5.87 %       ±2.69% ±3.59% ±4.68%
events/ee-listeners.js raw='true' listeners=50 n=5000000                        -0.14 %       ±1.05% ±1.40% ±1.83%
events/ee-once.js argc=0 n=20000000                                     ***     25.71 %       ±1.08% ±1.43% ±1.87%
events/ee-once.js argc=1 n=20000000                                     ***     24.94 %       ±0.77% ±1.02% ±1.33%
events/ee-once.js argc=4 n=20000000                                     ***     25.14 %       ±0.92% ±1.23% ±1.61%
events/ee-once.js argc=5 n=20000000                                     ***     25.63 %       ±0.90% ±1.21% ±1.58%

Be aware that when doing many comparisons the risk of a false-positive result increases.
In this case, there are 31 comparisons, you can thus expect the following amount of false-positive results:
  1.55 false positives, when considering a   5% risk acceptance (*, **, ***),
  0.31 false positives, when considering a   1% risk acceptance (**, ***),
  0.03 false positives, when considering a 0.1% risk acceptance (***)

streams

                                                                                         confidence improvement accuracy (*)    (**)   (***)
streams/creation.js kind='duplex' n=50000000                                                    ***      1.32 %       ±0.61%  ±0.81%  ±1.05%
streams/creation.js kind='readable' n=50000000                                                          -0.37 %       ±0.61%  ±0.81%  ±1.06%
streams/creation.js kind='transform' n=50000000                                                   *      3.83 %       ±3.24%  ±4.37%  ±5.78%
streams/creation.js kind='writable' n=50000000                                                  ***      2.83 %       ±0.55%  ±0.73%  ±0.95%
streams/destroy.js kind='duplex' n=1000000                                                              -0.08 %       ±0.64%  ±0.85%  ±1.11%
streams/destroy.js kind='readable' n=1000000                                                    ***      3.98 %       ±1.82%  ±2.42%  ±3.16%
streams/destroy.js kind='transform' n=1000000                                                            0.32 %       ±0.54%  ±0.72%  ±0.94%
streams/destroy.js kind='writable' n=1000000                                                    ***      2.57 %       ±0.60%  ±0.79%  ±1.03%
streams/iter-creation.js n=100000 type='pair' api='classic'                                              1.49 %       ±4.62%  ±6.15%  ±8.00%
streams/iter-creation.js n=100000 type='pair' api='iter'                                                 0.02 %       ±0.78%  ±1.04%  ±1.35%
streams/iter-creation.js n=100000 type='pair' api='webstream'                                           -0.31 %       ±1.61%  ±2.14%  ±2.78%
streams/iter-creation.js n=100000 type='readable' api='classic'                                         -0.15 %       ±5.01%  ±6.67%  ±8.69%
streams/iter-creation.js n=100000 type='readable' api='iter'                                             0.48 %       ±2.18%  ±2.90%  ±3.77%
streams/iter-creation.js n=100000 type='readable' api='webstream'                                       -0.29 %       ±1.71%  ±2.27%  ±2.96%
streams/iter-creation.js n=100000 type='transform' api='classic'                                         1.97 %       ±2.41%  ±3.21%  ±4.18%
streams/iter-creation.js n=100000 type='transform' api='webstream'                                       0.19 %       ±0.86%  ±1.14%  ±1.49%
streams/iter-creation.js n=100000 type='writable' api='classic'                                          1.07 %       ±2.10%  ±2.80%  ±3.64%
streams/iter-creation.js n=100000 type='writable' api='iter'                                             0.60 %       ±1.89%  ±2.51%  ±3.27%
streams/iter-creation.js n=100000 type='writable' api='webstream'                                       -0.18 %       ±1.23%  ±1.64%  ±2.13%
streams/pipe.js n=5000000                                                                       ***     13.92 %       ±1.41%  ±1.88%  ±2.45%
streams/readable-async-iterator.js sync='no' n=100000                                             *      2.45 %       ±1.86%  ±2.48%  ±3.24%
streams/readable-async-iterator.js sync='yes' n=100000                                                  -3.12 %       ±3.85%  ±5.15%  ±6.76%
streams/readable-bigread.js n=1000                                                                       0.14 %       ±2.98%  ±3.96%  ±5.16%
streams/readable-readall.js n=5000                                                                      -0.24 %       ±2.49%  ±3.32%  ±4.33%
streams/writable-manywrites.js len=1024 callback='no' writev='no' sync='no' n=100000                    -0.87 %       ±2.73%  ±3.64%  ±4.75%
streams/writable-manywrites.js len=1024 callback='no' writev='no' sync='yes' n=100000                    1.20 %       ±4.18%  ±5.57%  ±7.29%
streams/writable-manywrites.js len=1024 callback='no' writev='yes' sync='no' n=100000                    2.47 %       ±5.23%  ±6.98%  ±9.13%
streams/writable-manywrites.js len=1024 callback='no' writev='yes' sync='yes' n=100000                  -1.24 %       ±7.14%  ±9.50% ±12.36%
streams/writable-manywrites.js len=1024 callback='yes' writev='no' sync='no' n=100000             *      2.65 %       ±2.43%  ±3.23%  ±4.22%
streams/writable-manywrites.js len=1024 callback='yes' writev='no' sync='yes' n=100000           **      8.29 %       ±5.26%  ±7.00%  ±9.11%
streams/writable-manywrites.js len=1024 callback='yes' writev='yes' sync='no' n=100000                   2.01 %       ±4.22%  ±5.62%  ±7.31%
streams/writable-manywrites.js len=1024 callback='yes' writev='yes' sync='yes' n=100000           *      5.16 %       ±5.15%  ±6.85%  ±8.92%
streams/writable-manywrites.js len=32768 callback='no' writev='no' sync='no' n=100000                    2.67 %       ±3.56%  ±4.73%  ±6.16%
streams/writable-manywrites.js len=32768 callback='no' writev='no' sync='yes' n=100000            *      5.65 %       ±4.82%  ±6.42%  ±8.37%
streams/writable-manywrites.js len=32768 callback='no' writev='yes' sync='no' n=100000                  -0.76 %       ±1.95%  ±2.59%  ±3.37%
streams/writable-manywrites.js len=32768 callback='no' writev='yes' sync='yes' n=100000                  4.26 %       ±8.13% ±10.81% ±14.07%
streams/writable-manywrites.js len=32768 callback='yes' writev='no' sync='no' n=100000                  -1.57 %       ±3.42%  ±4.56%  ±5.94%
streams/writable-manywrites.js len=32768 callback='yes' writev='no' sync='yes' n=100000                  2.83 %       ±4.44%  ±5.91%  ±7.69%
streams/writable-manywrites.js len=32768 callback='yes' writev='yes' sync='no' n=100000                 -0.87 %       ±2.98%  ±3.96%  ±5.16%
streams/writable-manywrites.js len=32768 callback='yes' writev='yes' sync='yes' n=100000                 2.03 %       ±5.96%  ±7.93% ±10.32%

Be aware that when doing many comparisons the risk of a false-positive result increases.
In this case, there are 40 comparisons, you can thus expect the following amount of false-positive results:
  2.00 false positives, when considering a   5% risk acceptance (*, **, ***),
  0.40 false positives, when considering a   1% risk acceptance (**, ***),
  0.04 false positives, when considering a 0.1% risk acceptance (***)

Use a closure instead of a bound state object for the once() wrapper, keep the events object shape in removeListener() by storing undefined instead of deleting keys or reallocating the object, and outline the cold unhandled 'error' path from emit() and the max-listeners warning from _addListener() so both fit within V8 inlining budgets. ee-once +25%, ee-add-remove +12-22%, ee-listen-unique +7-15%, streams/pipe +13.9%. Signed-off-by: Matteo Collina <hello@matteocollina.com>

codecov · 2026-07-08T22:49:08Z

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.24%. Comparing base (1d87a24) to head (ce6c9ad).
⚠️ Report is 43 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #64373      +/-   ##
==========================================
- Coverage   90.25%   90.24%   -0.01%     
==========================================
  Files         741      741              
  Lines      240990   241196     +206     
  Branches    45402    45431      +29     
==========================================
+ Hits       217502   217666     +164     
- Misses      15066    15086      +20     
- Partials     8422     8444      +22

Files with missing lines	Coverage Δ
lib/events.js	`99.59% <100.00%> (+<0.01%)`	⬆️

... and 48 files with indirect coverage changes

🚀 New features to boost your workflow:

❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
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jasnell · 2026-07-09T13:43:17Z

+        configurable: true,
+      });
+    } catch {
+      // Continue regardless of error.


Can you expand this to explain why we're ignoring the error?

I know this isn't new but it's always bugged me that's it's not explained :-)

In the lines above we are trying to expand the stacktrace. If we can't for whatever reasons, we don't hide the original error.

@jasnell do you want me to update the comment?

nodejs-github-bot · 2026-07-09T16:58:57Z

CI: https://ci.nodejs.org/job/node-test-pull-request/74700/

nodejs-github-bot · 2026-07-10T23:47:06Z

CI: https://ci.nodejs.org/job/node-test-pull-request/74730/

nodejs-github-bot · 2026-07-11T22:32:47Z

CI: https://ci.nodejs.org/job/node-test-pull-request/74743/

nodejs-github-bot · 2026-07-11T22:32:49Z

CI: https://ci.nodejs.org/job/node-test-pull-request/74744/

nodejs-github-bot · 2026-07-12T13:15:12Z

CI: https://ci.nodejs.org/job/node-test-pull-request/74754/

nodejs-github-bot · 2026-07-13T08:50:11Z

Landed in 384d7a4

ronag · 2026-07-13T15:55:07Z

+        // the `events` object in the same shape and avoids both a `delete`
+        // (which would put the object into dictionary mode) and allocating
+        // a fresh object when the last listener is removed.
+        events[type] = undefined;


This can cause memory leaks when using unstructured event names, I.e random ids etc...

Restore shape mode for preallocated EventEmitters while deleting dynamic names from ordinary EventEmitters. Keeping every removed name as an undefined property causes unbounded memory growth when event names are unstructured. Refs: nodejs#64373 Signed-off-by: Matteo Collina <hello@matteocollina.com>

Restore shape mode for preallocated EventEmitters while deleting dynamic names from ordinary EventEmitters. Keeping every removed name as an undefined property causes unbounded memory growth when event names are unstructured. Refs: #64373 Signed-off-by: Matteo Collina <hello@matteocollina.com> PR-URL: #64475 Reviewed-By: Robert Nagy <ronagy@icloud.com> Reviewed-By: James M Snell <jasnell@gmail.com>

nodejs-github-bot added events Issues and PRs related to the events subsystem / EventEmitter. needs-ci PRs that need a full CI run. labels Jul 8, 2026

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mcollina added commit-queue Add this label to land a pull request using GitHub Actions. author ready PRs that have at least one approval, no pending requests for changes, and a CI started. and removed needs-ci PRs that need a full CI run. labels Jul 13, 2026

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nodejs-github-bot merged commit 384d7a4 into nodejs:main Jul 13, 2026
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mcollina deleted the events-optimize-once-removelistener branch July 13, 2026 16:02

mcollina mentioned this pull request Jul 13, 2026

events: avoid retaining removed event names #64475

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github-actions Bot mentioned this pull request Jul 14, 2026

[Vulnerability] nodejs/node: Denial of Service (Unbounded Memory Growth / Memory Leak) spaceraccoon/vulnerability-spoiler-alert#320

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