lib: avoid quadratic shift() in startup snapshot callback

[watilde]

Micro benchmark

$ cat bench-startup-snapshot-callbacks.js
'use strict';

function runWithShift(callbacks) {
  while (callbacks.length > 0) {
    const { 0: cb, 1: data } = callbacks.shift();
    cb(data);
  }
}

function runWithIndex(callbacks) {
  for (let i = 0; i < callbacks.length; i++) {
    const { 0: cb, 1: data } = callbacks[i];
    cb(data);
  }
  callbacks.length = 0;
}

function makeCallbacks(count) {
  return Array.from({ length: count }, (_, j) => [() => {}, j]);
}

function measure(fn, count, iterations) {
  // warmup
  for (let i = 0; i < 50; i++) fn(makeCallbacks(count));

  const start = performance.now();
  for (let i = 0; i < iterations; i++) fn(makeCallbacks(count));
  return performance.now() - start;
}

const COUNTS = [10, 100, 1_000, 10_000, 100_000];

console.log('--- shift() vs for+length=0 ---\n');

for (const count of COUNTS) {
  const iters = Math.max(10, Math.floor(1_000_000 / count));
  const shiftMs = measure(runWithShift, count, iters);
  const indexMs = measure(runWithIndex, count, iters);
  const ratio = shiftMs / indexMs;

  const countStr = `count=${String(count).padStart(6)}`;
  const shiftStr = `${shiftMs.toFixed(2).padStart(10)}ms`;
  const indexStr = `${indexMs.toFixed(2).padStart(8)}ms`;
  const ratioStr = `${ratio.toFixed(ratio >= 10 ? 0 : 1).padStart(5)}x faster`;

  console.log(`${countStr}  ${shiftStr}  vs  ${indexStr}   => ${ratioStr}`);
}

Result

$ ./node bench-startup-snapshot-callbacks.js
--- shift() vs for+length=0 ---

count=    10       58.22ms  vs     49.66ms   =>   1.2x faster
count=   100       42.62ms  vs     32.54ms   =>   1.3x faster
count=  1000       53.96ms  vs     30.92ms   =>   1.7x faster
count= 10000       60.50ms  vs     28.47ms   =>   2.1x faster
count=100000    46747.20ms  vs     50.64ms   =>   923x faster

[nodejs-github-bot]

Review requested:

• @nodejs/performance

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.62%. Comparing base (d44a71a) to head (100d584).
⚠️ Report is 72 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #62914   +/-   ##
=======================================
  Coverage   89.61%   89.62%           
=======================================
  Files         706      706           
  Lines      219203   219207    +4     
  Branches    41995    41999    +4     
=======================================
+ Hits       196445   196455   +10     
+ Misses      14663    14656    -7     
- Partials     8095     8096    +1     

Files with missing lines	Coverage Δ
lib/internal/v8/startup_snapshot.js	95.48% <100.00%> (+0.10%)	⬆️

... and 31 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[mcollina]

lgtm

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/72949/

[nodejs-github-bot]

Commit Queue failed

- Loading data for nodejs/node/pull/62914
✔  Done loading data for nodejs/node/pull/62914
----------------------------------- PR info ------------------------------------
Title      lib: avoid quadratic shift() in startup snapshot callback (#62914)
Author     Daijiro Wachi <daijiro.wachi@gmail.com> (@watilde)
Branch     watilde:startup-snapshot -> nodejs:main
Labels     performance, needs-ci, v8 module
Commits    1
 - lib: avoid quadratic shift() in startup snapshot callback
Committers 1
 - Daijiro Wachi <daijiro.wachi@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/62914
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Vinícius Lourenço Claro Cardoso <contact@viniciusl.com.br>
Reviewed-By: Ilyas Shabi <ilyasshabi94@gmail.com>
Reviewed-By: Gürgün Dayıoğlu <hey@gurgun.day>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
------------------------------ Generated metadata ------------------------------
PR-URL: https://github.com/nodejs/node/pull/62914
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Vinícius Lourenço Claro Cardoso <contact@viniciusl.com.br>
Reviewed-By: Ilyas Shabi <ilyasshabi94@gmail.com>
Reviewed-By: Gürgün Dayıoğlu <hey@gurgun.day>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
--------------------------------------------------------------------------------
   ℹ  This PR was created on Thu, 23 Apr 2026 17:49:26 GMT
   ✔  Approvals: 5
   ✔  - Stephen Belanger (@Qard): https://github.com/nodejs/node/pull/62914#pullrequestreview-4174486412
   ✔  - Vinícius Lourenço Claro Cardoso (@H4ad): https://github.com/nodejs/node/pull/62914#pullrequestreview-4177919400
   ✔  - Ilyas Shabi (@IlyasShabi): https://github.com/nodejs/node/pull/62914#pullrequestreview-4178825311
   ✔  - Gürgün Dayıoğlu (@gurgunday): https://github.com/nodejs/node/pull/62914#pullrequestreview-4179212038
   ✔  - Matteo Collina (@mcollina) (TSC): https://github.com/nodejs/node/pull/62914#pullrequestreview-4179356218
   ✘  GitHub CI failed with status: FAILURE
   ℹ  Last Full PR CI on 2026-04-27T09:04:48Z: https://ci.nodejs.org/job/node-test-pull-request/72949/
- Querying data for job/node-test-pull-request/72949/
✔  Build data downloaded
- Querying failures of job/node-test-commit/87332/
✔  Data downloaded
   ✘  1 failure(s) on the last Jenkins CI run
--------------------------------------------------------------------------------
   ✔  Aborted `git node land` session in /home/runner/work/node/node/.ncu

https://github.com/nodejs/node/actions/runs/24997533196

Landed in b6b6e96