stream: deprecate legacy prependListener fallback

[karan-lrn]

The prependListener method has been available on EventEmitter since Node.js v6.0.0 (April 2016). The fallback code that manipulated the internal _events object is no longer necessary.

This also removes tests that explicitly tested the fallback behavior.

[nodejs-github-bot]

Review requested:

• @nodejs/streams

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.70%. Comparing base (0dfdec9) to head (a603461).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #62435      +/-   ##
==========================================
- Coverage   89.71%   89.70%   -0.02%     
==========================================
  Files         695      695              
  Lines      214154   214224      +70     
  Branches    41009    41014       +5     
==========================================
+ Hits       192132   192167      +35     
- Misses      14075    14112      +37     
+ Partials     7947     7945       -2     

Files with missing lines	Coverage Δ
lib/internal/streams/legacy.js	94.89% <100.00%> (+0.44%)	⬆️

... and 40 files with indirect coverage changes

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[karan-lrn]

cc @nodejs/streams @mcollina @ronag

This PR removes the legacy prependListener fallback in lib/internal/streams/legacy.js. The fallback code manipulated the internal _events object to support Node.js versions before v6.0.0 (April 2016) where prependListener didn't exist.

Since Node.js v6 has been EOL for over 8 years, this cleanup removes ~15 lines of unnecessary code and the associated tests that explicitly tested the fallback behavior.

[mcollina]

It would be better to deprecate this first

[karan-lrn]

Hey @mcollina
Thanks for the feedback! I've updated the PR to add a runtime deprecation (DEP0205) instead of removing the code outright.

The deprecation warning now fires when piping to an EventEmitter that lacks the prependListener method. The fallback behavior is preserved for now, giving users time to migrate.

Once this lands and goes through a few releases, we can remove the fallback code entirely.

[mcollina]

lgtm

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/72172/

[nodejs-github-bot]

Commit Queue failed

- Loading data for nodejs/node/pull/62435
✔  Done loading data for nodejs/node/pull/62435
----------------------------------- PR info ------------------------------------
Title      stream: deprecate legacy prependListener fallback (#62435)
Author     Karan Mangtani <karanmangtani5@gmail.com> (@karan-lrn)
Branch     karan-lrn:stream-remove-prependlistener-hack -> nodejs:main
Labels     stream, semver-major, needs-ci
Commits    1
 - stream: deprecate piping to emitters without prependListener
Committers 1
 - Karan Mangtani <karan.mangtani@lrn.com>
PR-URL: https://github.com/nodejs/node/pull/62435
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Mattias Buelens <mattias@buelens.com>
------------------------------ Generated metadata ------------------------------
PR-URL: https://github.com/nodejs/node/pull/62435
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Mattias Buelens <mattias@buelens.com>
--------------------------------------------------------------------------------
   ℹ  This PR was created on Wed, 25 Mar 2026 20:25:37 GMT
   ✔  Approvals: 3
   ✔  - Matteo Collina (@mcollina) (TSC): https://github.com/nodejs/node/pull/62435#pullrequestreview-4023940838
   ✔  - James M Snell (@jasnell) (TSC): https://github.com/nodejs/node/pull/62435#pullrequestreview-4024660541
   ✔  - Mattias Buelens (@MattiasBuelens): https://github.com/nodejs/node/pull/62435#pullrequestreview-4025663529
   ✘  GitHub CI failed with status: FAILURE
   ℹ  Last Full PR CI on 2026-03-28T22:58:04Z: https://ci.nodejs.org/job/node-test-pull-request/72172/
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   ✘  3 failure(s) on the last Jenkins CI run
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   ✔  Aborted `git node land` session in /home/runner/work/node/node/.ncu

https://github.com/nodejs/node/actions/runs/23706198878

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/72191/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/72418/

[mcollina]

Can you rebase?

[karan-lrn]

@mcollina done!