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fix(minifier): prevent incorrect ??= transformation when member base is mutated #17472

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graphite-app merged 1 commit into from
Dec 30, 2025
Merged

fix(minifier): prevent incorrect ??= transformation when member base is mutated #17472

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34 changes: 34 additions & 0 deletions crates/oxc_minifier/src/peephole/remove_unused_expression.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -143,6 +143,10 @@ impl<'a> PeepholeOptimizations {
new_left_hand_expr,
ctx,
)
// Don't transform `x.y != null || (x = {}, x.y = 3)` to `x.y ??= (x = {}, 3)` because
// `??=` evaluates `x.y` (capturing `x`) before the RHS reassigns `x`.
// https://github.com/oxc-project/oxc/pull/16802#discussion_r2619369597
&& !Self::member_object_may_be_mutated(&assignment_expr.left, ctx)
{
assignment_expr.span = *logical_span;
assignment_expr.operator = AssignmentOperator::LogicalNullish;
Expand Down Expand Up @@ -861,6 +865,36 @@ mod test {
test_same("v = a != null || (a = b)");
test("void (x == null && y)", "x ?? y");

// https://github.com/oxc-project/oxc/pull/16802#discussion_r2619369597
// Don't transform to ??= when base object may be mutated, but ?? is safe
test(
"var x = {}; x.y != null || (x = {}, x.y = 3)",
"var x = {}; x.y ?? (x = {}, x.y = 3)",
);
test(
"var x = {}; x.y == null && (x = {}, x.y = 3)",
"var x = {}; x.y ?? (x = {}, x.y = 3)",
);
test(
"var x = {}; x.y != null || (a, x = {}, x.y = 3)",
"var x = {}; x.y ?? (a, x = {}, x.y = 3)",
);
test(
"var x = { y: {} }; x.y.z != null || (x.y = {}, x.y.z = 3)",
"var x = { y: {} }; x.y.z ?? (x.y = {}, x.y.z = 3)",
);
// Safe to transform to ??= when base object is not mutated
test("var x = {}; x.y != null || (foo(), x.y = 3)", "var x = {}; x.y ??= (foo(), 3)");
test(
"var x = {}; x.y != null || (new Foo(), x.y = 3)",
"var x = {}; x.y ??= (new Foo(), 3)",
);
// x is not mutated, only x.y.z is assigned (doesn't affect x)
test(
"var x = {}; x.y != null || (x.y.z = {}, x.y = 3)",
"var x = {}; x.y ??= (x.y.z = {}, 3)",
);

test("typeof x != 'undefined' && x", "");
test("typeof x == 'undefined' || x", "");
test("typeof x < 'u' && x", "");
Expand Down
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