Oxc minifier removes `if` guard and changes runtime behavior

[NullEnt1ty]

Describe the bug

Since upgrading to Vite 8, the default Oxc minifier appears to incorrectly optimize one of my functions and changes its runtime behavior. I reported the bug downstream in vitejs/vite#22234 but was told to report it here.

Here is a stripped down version of my code:

let rafId: number | undefined;

function foo() {
  if (rafId == null) {
    rafId = requestAnimationFrame(() => {
      console.log('callback');
    });
  }
}

Minified build output using oxc:

function e(){requestAnimationFrame(()=>{console.log(`callback`)})}

Minified build output using esbuild:

function c(){n==null&&(n=requestAnimationFrame(()=>{console.log("callback")}))}

The if (rafId == null) guard is gone entirely (when using oxc), which changes the behavior of the program. The original function schedules at most one pending animation frame at a time, while the minified output schedules a new one on every call.

Reproduction

The bug only happens when using "Transformer" with target >= 2021 and "Minify Syntax" checked.

https://playground.oxc.rs/?options=%7B%22run%22%3A%7B%22lint%22%3Afalse%2C%22formatter%22%3Afalse%2C%22transform%22%3Atrue%2C%22isolatedDeclarations%22%3Afalse%2C%22whitespace%22%3Afalse%2C%22mangle%22%3Afalse%2C%22compress%22%3Atrue%2C%22scope%22%3Atrue%2C%22symbol%22%3Atrue%2C%22cfg%22%3Afalse%7D%2C%22parser%22%3A%7B%22extension%22%3A%22tsx%22%2C%22allowReturnOutsideFunction%22%3Atrue%2C%22preserveParens%22%3Atrue%2C%22allowV8Intrinsics%22%3Atrue%2C%22semanticErrors%22%3Atrue%7D%2C%22linter%22%3A%7B%7D%2C%22formatter%22%3A%7B%22useTabs%22%3Afalse%2C%22tabWidth%22%3A2%2C%22endOfLine%22%3A%22lf%22%2C%22printWidth%22%3A80%2C%22singleQuote%22%3Afalse%2C%22jsxSingleQuote%22%3Afalse%2C%22quoteProps%22%3A%22as-needed%22%2C%22trailingComma%22%3A%22all%22%2C%22semi%22%3Atrue%2C%22arrowParens%22%3A%22always%22%2C%22bracketSpacing%22%3Atrue%2C%22bracketSameLine%22%3Afalse%2C%22objectWrap%22%3A%22preserve%22%2C%22singleAttributePerLine%22%3Afalse%7D%2C%22transformer%22%3A%7B%22target%22%3A%22es2022%22%2C%22useDefineForClassFields%22%3Atrue%2C%22experimentalDecorators%22%3Atrue%2C%22emitDecoratorMetadata%22%3Atrue%7D%2C%22isolatedDeclarations%22%3A%7B%22stripInternal%22%3Afalse%7D%2C%22codegen%22%3A%7B%22normal%22%3Atrue%2C%22jsdoc%22%3Atrue%2C%22annotation%22%3Atrue%2C%22legal%22%3Atrue%7D%2C%22compress%22%3A%7B%7D%2C%22mangle%22%3A%7B%22topLevel%22%3Atrue%2C%22keepNames%22%3Afalse%7D%2C%22controlFlow%22%3A%7B%22verbose%22%3Afalse%7D%2C%22inject%22%3A%7B%22inject%22%3A%7B%7D%7D%2C%22define%22%3A%7B%22define%22%3A%7B%7D%7D%7D&code=let+rafId%3A+number+%7C+undefined%3B%0A%0Afunction+foo%28%29+%7B%0A++if+%28rafId+%3D%3D+null%29+%7B%0A++++rafId+%3D+requestAnimationFrame%28%28%29+%3D%3E+%7B%0A++++++console.log%28%27callback%27%29%3B%0A++++%7D%29%3B%0A++%7D%0A%7D

Minimal Vite 8 project

https://stackblitz.com/edit/vitejs-vite-byj9asxu?file=src%2Fmain.ts

Steps to reproduce

npm run build:oxc to minify using oxc
npm run build:esbuild to minify using esbuild

You can check the build output in dist/assets/

System Info

System:
    OS: macOS 26.3
    CPU: (12) arm64 Apple M2 Max
    Memory: 1.55 GB / 32.00 GB
    Shell: 5.3.9 - /opt/homebrew/bin/bash
  Binaries:
    Node: 24.13.0 - /Users/dominique/.local/state/fnm_multishells/19928_1775543807061/bin/node
    Yarn: 1.22.22 - /Users/dominique/.local/state/fnm_multishells/19928_1775543807061/bin/yarn
    npm: 11.6.2 - /Users/dominique/.local/state/fnm_multishells/19928_1775543807061/bin/npm
    pnpm: 10.33.0 - /opt/homebrew/bin/pnpm
    bun: 1.3.11 - /opt/homebrew/bin/bun
    Deno: 2.7.11 - /opt/homebrew/bin/deno
  Browsers:
    Brave Browser: 144.1.86.142
    Chrome: 147.0.7727.56
    Firefox: 149.0
    Safari: 26.3
  npmPackages:
    @vitejs/plugin-react: 6.0.1 => 6.0.1
    vite: 8.0.1 => 8.0.1

Used Package Manager

npm

[Leonabcd123]

The problem seems to be the following:

if (rafId == null) { rafId = ... } is first minified to rafId == null && (rafId = ...):

oxc/crates/oxc_minifier/src/peephole/minimize_if_statement.rs

Lines 25 to 26 in 9b4d9f6

	// "if (a) b();" => "a && b();"
	e => (LogicalOperator::And, e),

Then rafId == null && (rafId = ...) is minified to rafId ??= ...:

oxc/crates/oxc_minifier/src/peephole/remove_unused_expression.rs

Line 126 in 9b4d9f6

// "a == null && (a = b)" => "a ??= b"

oxc/crates/oxc_minifier/src/peephole/remove_unused_expression.rs

Lines 154 to 157 in 9b4d9f6

	assignment_expr.operator = AssignmentOperator::LogicalNullish;
	*e = logical_right.take_in(ctx.ast);
	ctx.state.changed = true;
	return false;

And because rafId is considered unused (we don't mark it as read after converting to ??=, so it's only a write operation), that assignment will be removed (meaning that we remove everything to the left of the = symbol) by remove_unused_assignment_expr:

oxc/crates/oxc_minifier/src/peephole/remove_unused_expression.rs

Lines 641 to 644 in 9b4d9f6

	if symbol_value.read_references_count > 0 {
	return false;
	}
	*e = assign_expr.right.take_in(ctx.ast);

So we're left with the right side of the assignment (in this case, requestAnimationFrame(()=>{console.log(`callback`)})).

Fix

We could mark it as read so it won't be considered unused:

diff --git a/crates/oxc_minifier/src/peephole/remove_unused_expression.rs b/crates/oxc_minifier/src/peephole/remove_unused_expression.rs
index fc914881a..16f93c557 100644
--- a/crates/oxc_minifier/src/peephole/remove_unused_expression.rs
+++ b/crates/oxc_minifier/src/peephole/remove_unused_expression.rs
@@ -150,6 +150,7 @@ impl<'a> PeepholeOptimizations {
                                     // https://github.com/oxc-project/oxc/pull/16802#discussion_r2619369597
                                     && !Self::member_object_may_be_mutated(&assignment_expr.left, ctx)
                                 {
+                                    Self::mark_assignment_target_as_read(&assignment_expr.left, ctx);
                                     assignment_expr.span = *logical_span;
                                     assignment_expr.operator = AssignmentOperator::LogicalNullish;
                                     *e = logical_right.take_in(ctx.ast);

Not entering the if statement for the second step (converting to ??=):

oxc/crates/oxc_minifier/src/peephole/remove_unused_expression.rs

Lines 139 to 151 in 9b4d9f6

	if ctx.supports_feature(ESFeature::ES2021LogicalAssignmentOperators)
	&& let Expression::AssignmentExpression(assignment_expr) =
	logical_right
	&& assignment_expr.operator == AssignmentOperator::Assign
	&& Self::has_no_side_effect_for_evaluation_same_target(
	&assignment_expr.left,
	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)

Will have the same effect, because then rafId == null && (rafId = ...) will be minified to rafId ?? (rafId = ...):

oxc/crates/oxc_minifier/src/peephole/remove_unused_expression.rs

Lines 160 to 165 in 9b4d9f6

	*e = ctx.ast.expression_logical(
	*logical_span,
	new_left_hand_expr.take_in(ctx.ast),
	LogicalOperator::Coalesce,
	logical_right.take_in(ctx.ast),
	);

which will get converted to rafId ??= ... by try_compress_logical_expression_to_assignment_expression, which also marks it as read (which is needed so remove_unused_assignment_expr won't remove it):

oxc/crates/oxc_minifier/src/peephole/minimize_logical_expression.rs

Line 247 in 9b4d9f6

Self::mark_assignment_target_as_read(&assignment_expr.left, ctx);