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events: avoid cloning listeners array on every emit#62261

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gurgunday wants to merge 1 commit intonodejs:mainfrom
gurgunday:perf/ee-emit-copy
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events: avoid cloning listeners array on every emit#62261
gurgunday wants to merge 1 commit intonodejs:mainfrom
gurgunday:perf/ee-emit-copy

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@gurgunday gurgunday commented Mar 15, 2026 •
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This PR does 2 improvements:

  • Do not clone the listeners on each emit, clone when we're either adding or removing listeners
  • Even when we're adding or removing listeners, if we're not currently emitting, avoid cloning again

Results:

Branch:

./node benchmark/run.js --filter ee-emit events
events/ee-emit.js
events/ee-emit.js listeners=1 argc=0 n=2000000: 91,903,668.22922815
> events/ee-emit.js listeners=5 argc=0 n=2000000: 31,178,549.48221315
> events/ee-emit.js listeners=10 argc=0 n=2000000: 18,218,586.602051415
events/ee-emit.js listeners=1 argc=2 n=2000000: 97,625,065.27455927
> events/ee-emit.js listeners=5 argc=2 n=2000000: 30,786,518.583512284
> events/ee-emit.js listeners=10 argc=2 n=2000000: 17,759,695.723843552
events/ee-emit.js listeners=1 argc=4 n=2000000: 95,971,210.17248763
> events/ee-emit.js listeners=5 argc=4 n=2000000: 30,290,753.36889973
> events/ee-emit.js listeners=10 argc=4 n=2000000: 17,108,737.480991445
events/ee-emit.js listeners=1 argc=10 n=2000000: 93,870,086.24126433
> events/ee-emit.js listeners=5 argc=10 n=2000000: 28,639,532.56158767
> events/ee-emit.js listeners=10 argc=10 n=2000000: 16,616,102.896280617

main:

./node benchmark/run.js --filter ee-emit events
events/ee-emit.js
events/ee-emit.js listeners=1 argc=0 n=2000000: 92,364,529.17596893
> events/ee-emit.js listeners=5 argc=0 n=2000000: 29,932,147.413670897
> events/ee-emit.js listeners=10 argc=0 n=2000000: 15,313,442.157135993
events/ee-emit.js listeners=1 argc=2 n=2000000: 98,371,745.94257125
> events/ee-emit.js listeners=5 argc=2 n=2000000: 29,668,674.924437966
> events/ee-emit.js listeners=10 argc=2 n=2000000: 15,159,257.397702452
events/ee-emit.js listeners=1 argc=4 n=2000000: 96,956,966.8137633
> events/ee-emit.js listeners=5 argc=4 n=2000000: 29,187,937.01091416
> events/ee-emit.js listeners=10 argc=4 n=2000000: 14,862,263.968670348
events/ee-emit.js listeners=1 argc=10 n=2000000: 94,272,546.83507264
> events/ee-emit.js listeners=5 argc=10 n=2000000: 27,359,329.00041816
> events/ee-emit.js listeners=10 argc=10 n=2000000: 11,452,222.952059666

@nodejs-github-bot nodejs-github-bot added events Issues and PRs related to the events subsystem / EventEmitter. needs-ci PRs that need a full CI run. labels Mar 15, 2026
@gurgunday gurgunday added the performance Issues and PRs related to the performance of Node.js. label Mar 15, 2026
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codecov bot commented Mar 15, 2026 •
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Codecov Report

❌ Patch coverage is 93.33333% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.67%. Comparing base (8ccbe8e) to head (d335209).
⚠️ Report is 7 commits behind head on main.

Files with missing lines Patch % Lines
lib/events.js 93.33% 3 Missing ⚠️
Additional details and impacted files 
@@            Coverage Diff             @@
##             main   #62261      +/-   ##
==========================================
- Coverage   89.68%   89.67%   -0.02%     
==========================================
  Files         676      676              
  Lines      206495   206582      +87     
  Branches    39537    39559      +22     
==========================================
+ Hits       185204   185246      +42     
- Misses      13435    13459      +24     
- Partials     7856     7877      +21
Files with missing lines Coverage Δ
lib/events.js 99.59% <93.33%> (-0.24%) ⬇️

... and 42 files with indirect coverage changes

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mertcanaltin
mertcanaltin approved these changes Mar 15, 2026
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lgtm

@atlowChemi
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atlowChemi commented Mar 15, 2026

@nodejs/events cc

@atlowChemi atlowChemi added the needs-benchmark-ci PR that need a benchmark CI run. label Mar 15, 2026
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events Issues and PRs related to the events subsystem / EventEmitter. needs-benchmark-ci PR that need a benchmark CI run. needs-ci PRs that need a full CI run. performance Issues and PRs related to the performance of Node.js.

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