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fs: accept all valid utf8 values in fast paths#62304

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wdskuki wants to merge 1 commit intonodejs:mainfrom
wdskuki:fix-utf8-fast-paths
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fs: accept all valid utf8 values in fast paths#62304
wdskuki wants to merge 1 commit intonodejs:mainfrom
wdskuki:fix-utf8-fast-paths

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@wdskuki
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@wdskuki wdskuki commented Mar 18, 2026

This PR fixes issue #49888 by accepting all valid UTF8 encoding variants (utf8, utf-8, UTF8, UTF-8) in fs.readFileSync and fs.writeFileSync fast paths.

Changes

  • Modified lib/fs.js to accept UTF8 and UTF-8 (uppercase) in addition to utf8 and utf-8
  • Added test case to verify the fix

Testing

  • Added test/parallel/test-fs-utf8-fast-path-casing.js

Fixes: #49888

Accept UTF8 and UTF-8 (uppercase) in fs.readFileSync and
fs.writeFileSync fast paths, in addition to utf8 and utf-8.

Fixes: nodejs#49888
@nodejs-github-bot nodejs-github-bot added fs Issues and PRs related to the fs subsystem / file system. needs-ci PRs that need a full CI run. labels Mar 18, 2026
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@mcollina mcollina left a comment

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lgtm

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@jasnell jasnell left a comment

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LGTM modulo the linting issues in the test

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codecov bot commented Mar 19, 2026

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.68%. Comparing base (9fc6b64) to head (7f98286).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files
@@           Coverage Diff           @@
##             main   #62304   +/-   ##
=======================================
  Coverage   89.68%   89.68%           
=======================================
  Files         676      676           
  Lines      206575   206577    +2     
  Branches    39549    39553    +4     
=======================================
+ Hits       185262   185274   +12     
+ Misses      13446    13445    -1     
+ Partials     7867     7858    -9     
Files with missing lines Coverage Δ
lib/fs.js 98.19% <100.00%> (+<0.01%) ⬆️

... and 34 files with indirect coverage changes

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fs: utf8 fast paths don't accept all valid utf8 values

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