fix(minifier): prevent incorrect array.join("") to string concat for nullable elements#11546
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veeceey wants to merge 4 commits intoswc-project:mainfrom
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fix(minifier): prevent incorrect array.join("") to string concat for nullable elements#11546veeceey wants to merge 4 commits intoswc-project:mainfrom
veeceey wants to merge 4 commits intoswc-project:mainfrom
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…mization
Array.prototype.join converts null/undefined elements to empty strings,
but string concatenation with + converts them to "null"/"undefined".
This caused incorrect output when compress.evaluate was enabled for
arrays containing expressions that could evaluate to undefined/null.
For example:
["abc", cond ? undefined : "def"].join("") should produce "abc" when
cond is true, but was being converted to "abc" + (cond ? void 0 : "def")
which produces "abcundefined".
The fix bails out of the partial join optimization when non-literal
expressions that aren't guaranteed to be strings are present in the
array with an empty separator.
Closes swc-project#11545
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Binary Sizes
Commit: 0a09de5 |
kdy1
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Feb 15, 2026
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CI failed. Can you fix it?
You can do UPDATE=1 cargo test from the minifier directory or swc directory to update the test refernces
…uard The previous approach using is_definitely_string was too conservative and blocked the join-to-concat optimization for function calls, variables, and other expressions that terser also optimizes. This caused 8 fixture test failures. Replace with may_produce_null_or_undefined which only bails out when an expression obviously produces null/undefined (direct null/undefined/void literals, or conditional expressions with nullable branches). This still prevents the incorrect transformation from swc-project#11545 while preserving terser-compatible behavior for other cases.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Author
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Fixed the clippy issue — changed |
Author
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CI is green now after the clippy fix — switched to is_some_and() as suggested. Would appreciate a re-review when you get a chance! |
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Fixes #11545
When
compress.evaluateis enabled,compress_array_join_partialconverts[...].join("")to string concatenation with+. This is incorrect when array elements can beundefinedornull, becauseArray.prototype.joinconverts these to empty strings, while+converts them to"undefined"/"null".For example:
The fix adds a check in the empty-separator path of
compress_array_join_partialthat bails out when non-literal expressions aren't guaranteed to be strings (checked viais_definitely_string). This prevents the semantics-changing transformation while still allowing the optimization for safe cases like template literals and known string concatenations.All 471 exec tests pass, including 2 new tests covering the exact scenario from the issue.