fix(ecmascript): treat update expressions as unconditionally side-effectful#21456
Merged
Merged
Conversation
Member
Author
How to use the Graphite Merge QueueAdd either label to this PR to merge it via the merge queue:
You must have a Graphite account in order to use the merge queue. Sign up using this link. An organization admin has enabled the Graphite Merge Queue in this repository. Please do not merge from GitHub as this will restart CI on PRs being processed by the merge queue. This stack of pull requests is managed by Graphite. Learn more about stacking. |
Merging this PR will not alter performance
Comparing 
Footnotes
|
Dunqing
marked this pull request as ready for review
April 15, 2026 04:55
Member
Author
|
I remember the original behavior comes from Rolldown for some tests; maybe we need a fix in Rolldown as well. |
Dunqing
force-pushed
the
fix/update-expression-side-effects
branch
from
April 15, 2026 07:46
0879511 to
1860cc7
Compare
sapphi-red
approved these changes
Apr 15, 2026
Member
Author
Merge activity
|
…ectful (#21456) ## Summary `obj.prop++` performs an implicit `GetValue` + `ToNumeric` + `PutValue`. The `ToNumeric` coercion alone can invoke `valueOf`/`Symbol.toPrimitive` on the old value, and the read/write may trigger getters, setters, or Proxy traps. Previously `UpdateExpression::may_have_side_effects` only checked `property_write_side_effects()` and treated the update as free when disabled — letting the minifier drop `counter.value++` even when `counter` is externally observable. ## Fix Return `true` unconditionally for `UpdateExpression`. Terser, esbuild, Rollup, and SWC all hardcode `++`/`--` as unconditionally side-effectful — this matches their behavior, and mirrors how compound assignments like `a.b += 1` are already handled one impl above (`AssignmentExpression`). ## Impact Under rolldown's tree-shake config (`propertyWriteSideEffects: false`), these are now correctly preserved instead of being dropped: ```js counter.value++; (class { static { ++counter.count; } }); class A { [counter.another++] = 123; } ``` ## Reported rolldown/rolldown#9094 ([comment](rolldown/rolldown#9094 (comment)))
graphite-app
Bot
force-pushed
the
fix/update-expression-side-effects
branch
from
April 15, 2026 14:13
1860cc7 to
d7a359a
Compare
Dunqing
added a commit
that referenced
this pull request
Apr 16, 2026
### 💥 BREAKING CHANGES - 24fb7eb allocator: [**BREAKING**] Rename `Box` and `Vec` methods (#21395) (overlookmotel) ### 🚀 Features - ce5072d parser: Support `turbopack` magic comments (#20803) (Kane Wang) - f5deb55 napi/transform: Expose `optimizeConstEnums` and `optimizeEnums` options (#21388) (Dunqing) - 24b03de data_structures: Introduce `NonNullConst` and `NonNullMut` pointer types (#21425) (overlookmotel) ### 🐛 Bug Fixes - d7a359a ecmascript: Treat update expressions as unconditionally side-effectful (#21456) (Dunqing) - 56af2f4 transformer/async-to-generator: Correct scope of inferred named FE in async-to-generator (#21458) (Dunqing) - b3ed467 minifier: Avoid illegal `var;` when folding unused arguments copy loop (#21421) (fazba) - b0e8f13 minifier: Preserve `var` inside `catch` with same-named parameter (#21366) (Dunqing) - 4fb73a7 transformer/typescript: Preserve execution order for accessor with `useDefineForClassFields: false` (#21369) (Dunqing) ### ⚡ Performance - da3cc16 parser: Refactor out `LexerContext` (#21275) (Ulrich Stark) ### 📚 Documentation - c5b19bb allocator: Reformat comments in `Arena` (#21448) (overlookmotel) - 091e88e lexer: Update doc comment about perf benefit of reading through references (#21423) (overlookmotel) - 922cbee allocator: Remove references to "bump" from comments (#21397) (overlookmotel) Co-authored-by: Dunqing <[email protected]>
This was referenced Apr 16, 2026
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
2 participants
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
Summary
obj.prop++performs an implicitGetValue+ToNumeric+PutValue. TheToNumericcoercion alone can invokevalueOf/Symbol.toPrimitiveon the old value, and the read/write may trigger getters, setters, or Proxy traps.Previously
UpdateExpression::may_have_side_effectsonly checkedproperty_write_side_effects()and treated the update as free when disabled — letting the minifier dropcounter.value++even whencounteris externally observable.Fix
Return
trueunconditionally forUpdateExpression. Terser, esbuild, Rollup, and SWC all hardcode++/--as unconditionally side-effectful — this matches their behavior, and mirrors how compound assignments likea.b += 1are already handled one impl above (AssignmentExpression).Impact
Under rolldown's tree-shake config (
propertyWriteSideEffects: false), these are now correctly preserved instead of being dropped:Reported
rolldown/rolldown#9094 (comment)