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stream: deprecate legacy prependListener fallback#62435

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karan-lrn wants to merge 1 commit intonodejs:mainfrom
karan-lrn:stream-remove-prependlistener-hack
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stream: deprecate legacy prependListener fallback#62435
karan-lrn wants to merge 1 commit intonodejs:mainfrom
karan-lrn:stream-remove-prependlistener-hack

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@karan-lrn karan-lrn commented Mar 25, 2026

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.

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nodejs-github-bot commented Mar 25, 2026

Review requested:

  • @nodejs/streams

@nodejs-github-bot nodejs-github-bot added needs-ci PRs that need a full CI run. stream Issues and PRs related to the stream subsystem. labels Mar 25, 2026
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codecov bot commented Mar 25, 2026 •
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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.69%. Comparing base (0d7e4b1) to head (dea878a).
⚠️ Report is 19 commits behind head on main.

Additional details and impacted files 
@@           Coverage Diff            @@
##             main   #62435    +/-   ##
========================================
  Coverage   89.68%   89.69%            
========================================
  Files         676      678     +2     
  Lines      206738   207206   +468     
  Branches    39594    39726   +132     
========================================
+ Hits       185414   185854   +440     
+ Misses      13467    13447    -20     
- Partials     7857     7905    +48
Files with missing lines Coverage Δ
lib/internal/streams/legacy.js 94.89% <100.00%> (+0.44%) ⬆️

... and 66 files with indirect coverage changes

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karan-lrn commented Mar 26, 2026

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.

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mcollina requested changes Mar 26, 2026
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It would be better to deprecate this first

@karan-lrn karan-lrn force-pushed the stream-remove-prependlistener-hack branch from 73f182e to dbb2a5f Compare March 27, 2026 11:04
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karan-lrn commented Mar 27, 2026

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.

@karan-lrn
stream: deprecate piping to emitters without prependListener
dea878a 
The prependListener method has been available on EventEmitter since
Node.js v6.0.0 (April 2016). The internal fallback code that manually
manipulates the _events object was intended for pre-v6 compatibility.

This adds a runtime deprecation warning (DEP0205) when piping to an
EventEmitter that lacks the prependListener method. The fallback
behavior is preserved for now but will be removed in a future version.

PR-URL: https://github.com/nodejs/node/pull/XXXXX
Refs: https://github.com/nodejs/node/issues/XXXXX
@karan-lrn karan-lrn force-pushed the stream-remove-prependlistener-hack branch from dbb2a5f to dea878a Compare March 27, 2026 11:07
@karan-lrn karan-lrn requested a review from mcollina March 27, 2026 11:10
@karan-lrn karan-lrn changed the title stream: remove legacy prependListener fallback stream: deprecate legacy prependListener fallback Mar 27, 2026
@mcollina mcollina added the semver-major PRs that contain breaking changes and should be released in the next major version. label Mar 27, 2026
mcollina
mcollina approved these changes Mar 27, 2026
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lgtm

@mcollina mcollina added the request-ci Add this label to start a Jenkins CI on a PR. label Mar 27, 2026
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@aduh95 aduh95 removed the request-ci Add this label to start a Jenkins CI on a PR. label Mar 28, 2026
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nodejs-github-bot commented Mar 28, 2026

CI: https://ci.nodejs.org/job/node-test-pull-request/72143/

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@mcollina mcollina mcollina approved these changes

@jasnell jasnell jasnell approved these changes

@MattiasBuelens MattiasBuelens MattiasBuelens approved these changes

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