[Donny/강동윤 (kdy1)]

The minifier replaced a property read of trimLeft or trimRight on a string literal with void 0. Both are standard String.prototype methods (ECMA-262 Annex B aliases of trimStart and trimEnd), so "".trimLeft is a function, not undefined. This broke feature detection such as core-js "".trimLeft !== String.prototype.trimStart.

Pure::optimize_member_expr folds a string literal property read to undefined when the property name is not in STRING_SYMBOLS or OBJECT_SYMBOLS. STRING_SYMBOLS had trimStart and trimEnd, but not trimLeft and trimRight.

This PR adds trimLeft and trimRight to STRING_SYMBOLS. It also adds the fixture tests/fixture/issues/12570.

Validation, run locally on the changed code:

• ./scripts/test.sh in crates/swc_ecma_minifier: 2219 passed, 0 failed.
• ./scripts/exec.sh in crates/swc_ecma_minifier: 2128 passed, 0 failed.
• cargo clippy -p swc_ecma_minifier --all-targets -- -D warnings: no warnings.

CI was not run locally.

Closes #12570

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Latest commit: d87185d

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[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-20T04:23:55.191546Z	d87185d	PR opened
🔒 Security Review	✅ Completed 2026-10-20T04:24:25.437123Z	d87185d	PR opened

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[codspeed]

Merging this PR will not alter performance

⚠️ 30 benchmarks spent significant time in system calls

System calls cannot be consistently instrumented, so they are not included in the measure, which understates the real cost. Please switch to the Walltime instrument to accurately measure system calls.

Measurement and system calls

• es/large/minify/libraries/lodash
• es/large/minify/libraries/d3
• es/parser/backbone
• es/large/minify/libraries/jquery
• es/large/minify/libraries/three
• es/minifier/libs/echarts
• es/minifier/libs/lodash
• es/large/minify/libraries/terser
• es/minifier/libs/vue
• es/large/minify/libraries/moment
• …and 20 more

ℹ️ 2 benchmarks have no flame graph

The flamegraph could not be generated because no execution data was recorded when the benchmark was run. Ensure that the benchmark has not been optimized out.

• common/arena/iter-sparse/100000
• common/arena/lookup/100000

✅ 200 untouched benchmarks
⏩ 67 skipped benchmarks¹

---

_{Comparing kdy1/fix-issue-12570 (d87185d) with main (48586cb)}

Footnotes

1. 67 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩