fix: prevent style={undefined} from destroying computed styles in non-[style] targets

[YevheniiKotyrlo]

Summary

Fixes remaining Object.assign calls in deepMergeConfig that allow style={undefined} to destroy computed NativeWind styles on non-["style"] targets.

Problem

PR #224 fixed the most common code path (config.target = ["style"]), but two Object.assign({}, left, right) calls remain unfixed at lines 362 and 365.

Object.assign copies all enumerable own properties from right, including those with value undefined. When a component passes contentContainerStyle={undefined} alongside contentContainerClassName, the computed styles from left are overwritten with undefined.

Affected components:

• ScrollView — contentContainerClassName + contentContainerStyle={undefined}
• FlatList — contentContainerClassName / columnWrapperClassName + *Style={undefined}
• VirtualizedList — contentContainerClassName + contentContainerStyle={undefined}
• Any custom styled() with non-default target mappings (e.g. { className: { target: "style" } })

This is a common pattern in reusable components where style props are optional:

function MyScrollView({ contentContainerStyle, ...props }: ScrollViewProps) {
  return (
    <ScrollView
      contentContainerClassName="p-4"
      contentContainerStyle={contentContainerStyle}
      {...props}
    />
  );
}
// <MyScrollView /> — contentContainerStyle is implicitly undefined, className styles lost

Native-only — web uses array concatenation (no Object.assign).

Solution

Added a mergeDefinedProps helper that replaces both Object.assign({}, left, right) calls. It spreads left into a new object and iterates right via for-in, skipping keys with undefined values.

function mergeDefinedProps(
  left: Record<string, any> | undefined,
  right: Record<string, any>,
) {
  const result = left ? { ...left } : {};
  for (const key in right) {
    if (right[key] !== undefined) {
      result[key] = right[key];
    }
  }
  return result;
}

This covers all config.target types (array, string, false) at all recursion depths, and still allows explicit null values to override (only undefined is skipped). Uses for-in instead of Object.keys() to avoid intermediate array allocation. Both left and right are always plain object literals (JSX props or calculateProps output) — no prototype chain concerns.

Performance

Benchmarked 15 implementation variants of mergeDefinedProps in isolation (2M iterations, best of 5 runs) across 6 scenarios matching real component renders, then validated the winner in the full deepMergeConfig context (1M iterations each):

Scenario	Original	Fixed	Ratio
ScrollView + style={undefined} (the bug)	129ms	113ms	0.87x (13% faster)
ScrollView + valid style	125ms	122ms	0.97x (same)
FlatList + many props + undefined styles	320ms	302ms	0.94x (6% faster)
Custom styled() + style={undefined}	172ms	161ms	0.94x (6% faster)

The fix is faster than the original Object.assign across all real-world scenarios.

Verification

• yarn typecheck — pass
• yarn lint — pass
• yarn build — pass
• yarn test — 977 passed, 3 failed (pre-existing babel plugin failures, same on main)

Added 8 test cases to className-with-style.test.tsx covering all config.target types:

• Path A (["style"]): style={undefined} and style={null} preserve computed styles
• Path B (["contentContainerStyle"]): ScrollView and FlatList with contentContainerStyle={undefined} preserve computed styles; valid contentContainerStyle merges correctly; omitted prop preserves styles
• Path B (string target): custom styled() with { target: "style" } preserves styles with style={undefined}
• Real-world: optional contentContainerStyle prop forwarding (implicitly undefined) preserves computed styles

Reproduction

Visual reproduction repo with before/after screenshots: nativewind-style-undefined-repro

Before (3.0.4 unpatched)	After (patched)

Related

• Completes the fix started in Feat/deep merge config optimizations #224
• Related to Bug: Cannot set className and style at the same time in v5 #233 (last comment reports style={undefined} still breaking after Feat/deep merge config optimizations #224)