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perf: Only extract source map comments at the end of the file by liuxingbaoyu · Pull Request #18039 · babel/babel · GitHub

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perf: Only extract source map comments at the end of the file - #18039

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nicolo-ribaudo merged 2 commits into
babel:mainfrom
liuxingbaoyu:perf-input-map
Jun 16, 2026
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perf: Only extract source map comments at the end of the file#18039
nicolo-ribaudo merged 2 commits into
babel:mainfrom
liuxingbaoyu:perf-input-map

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liuxingbaoyu commented May 31, 2026 •
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Q                       A
Fixed Issues? Fixes #1, Fixes #2
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass? Yes
Documentation PR Link
Any Dependency Changes?
License MIT
  1. Only search for the input source map when source maps are needed.
  2. Only search the top-level source map.
  3. Print an error log when loading fails.

Potential impact:
Currently, if users do not enable sourceMaps, source map comments in the code will be retained.
If some tools directly concatenate the source code and map within functions, they will no longer be loaded. (However, we only load one source map anyway.)

This greatly improves core performance: ~20%

PS F:\babel\benchmark\babel-core\real-case-preset-env-typescript> node .\babel-parser-tokenizer-index.mjs         
current: 86.54 ops/sec ±1.7% (12ms)
PS F:\babel\benchmark\babel-core\real-case-preset-env-typescript> node .\babel-parser-tokenizer-index.mjs
current: 104 ops/sec ±1.38% (9.587ms)

Closes #16874

liuxingbaoyu added PR: Performance 🏃‍♀️ A type of pull request used for our changelog categories pkg: core area: sourcemaps labels May 31, 2026

babel-bot commented May 31, 2026 •
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Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/61771

chatgpt-codex-connector Bot reviewed May 31, 2026
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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

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Comment thread packages/babel-core/src/transformation/normalize-file.ts

pkg-pr-new Bot commented May 31, 2026 •
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commit: 022060e

nicolo-ribaudo reviewed May 31, 2026
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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

Technically if a source map comment is followed by code we should ignore it.

So it should be enough to scan from the end, and only the trailing comments.

Maybe we could even start doing it at the parser level at some point.

https://tc39.es/ecma426/#sec-JavaScriptExtractSourceMapURL

nicolo-ribaudo added PR: Spec Compliance 👓 A type of pull request used for our changelog categories PR: Performance 🏃‍♀️ A type of pull request used for our changelog categories and removed PR: Needs Docs PR: Performance 🏃‍♀️ A type of pull request used for our changelog categories labels Jun 14, 2026
nicolo-ribaudo changed the title perf: Improve the performance of processing source maps perf: Only extract source map comments at the end of the file Jun 14, 2026

nicolo-ribaudo commented Jun 14, 2026

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@liuxingbaoyu I pushed a commit to further limit which comments we look at, so that we match the spec :)

nicolo-ribaudo merged commit b71c35a into babel:main Jun 16, 2026
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