Fix null when accessing document.body

[mattgperry]

Add optional chaining (?.) and nullish coalescing (|| 0) when accessing document.body.scrollLeft and document.body.scrollTop to prevent TypeError when document.body is temporarily null during heavy DOM manipulation or aggressive reflows.

Fixes #3348

[claude]

Pull Request Review

Thanks for addressing issue #3348! This is a solid defensive fix for a rare but real edge case. Here's my detailed review:

✅ Code Quality & Best Practices

Strengths:

• Defensive programming: The optional chaining (?.) and nullish coalescing (|| 0) pattern is exactly right for this edge case
• Minimal change: The fix is surgical - only touching the specific lines that need protection
• Follows existing patterns: Uses the same fallback chain pattern (documentElement || body || 0) that was already in place
• Good test coverage: Three well-structured test cases covering the main scenarios

📝 Test Quality

The test file is comprehensive and well-written:

1. returns scroll position from documentElement - Tests the primary/happy path
2. does not throw when document.body is null - Tests the exact bug scenario from [BUG] Missing null check in DocumentProjectionNode #3348
3. falls back to body scroll when documentElement scroll is 0 - Tests the fallback behavior

Test structure strengths:

• Proper cleanup with afterEach to restore document.body
• Uses Object.defineProperty to mock read-only DOM properties correctly
• Clear comments explaining what each test does
• Tests both the non-throwing behavior AND the correct return value

🔍 Potential Issues & Concerns

1. Test implementation concern (Minor)

The test uses DocumentProjectionNode.create(window) but I don't see a static create method in the DocumentProjectionNode export. The createProjectionNode function returns a class, not an instance.

Expected pattern:

// DocumentProjectionNode is a class returned by createProjectionNode
const node = new DocumentProjectionNode()

Or if there's a factory:

// If there's a create method on the class
const node = DocumentProjectionNode.create(window)

Recommendation: Please verify the tests actually run and pass. If they don't compile/run, the instantiation pattern needs correction.

2. Edge case consideration (Very Minor)

The current implementation:

x: document.documentElement.scrollLeft || document.body?.scrollLeft || 0

This works correctly, but there's a subtle difference between undefined and 0. If document.body exists but scrollLeft is 0, the first || will short-circuit. This is actually fine for scroll positions since 0 is a valid value meaning "not scrolled", but it's worth noting that the fallback only happens when:

• document.documentElement.scrollLeft is 0 or falsy
• AND document.body is not null
• AND document.body.scrollLeft is truthy

The logic is correct for the use case, just documenting the behavior.

🔒 Security Concerns

None identified. This is a defensive null check with no security implications.

⚡ Performance Considerations

Excellent - no performance impact:

• Optional chaining (?.) is a compile-time feature (transpiled to simple null checks)
• No new function calls or allocations
• The || 0 fallback is a simple truthy check
• This code runs during scroll measurement which is already performance-critical, and the fix adds negligible overhead

🧪 Test Coverage

Strong coverage for the specific bug fix:

• ✅ Primary path (documentElement)
• ✅ Fallback path (body)
• ✅ Null body path (the bug scenario)

Possible additional test (optional):

• Test when document.body exists but scrollLeft/scrollTop are actually 0 (not undefined) - to verify we don't accidentally fall back to the final || 0 when body scroll is legitimately 0

📋 Summary & Recommendation

This is a high-quality defensive fix that addresses a real production issue. The implementation is clean, minimal, and follows best practices.

Before merging, please verify:

1. ✅ The tests actually run and pass (yarn test in packages/motion-dom)
2. ✅ The DocumentProjectionNode.create(window) pattern is correct for the codebase

Rating: 9/10 - Excellent fix with comprehensive tests. Only concern is verifying the test instantiation pattern is valid.

🎯 Specific Questions for Author

1. Have you run the tests locally to confirm they pass?
2. Is there a static create method on the returned class, or should the tests use new DocumentProjectionNode()?

Great work on fixing this rare but important edge case! 🚀

[claude]

PR Review: Fix null when accessing document.body

Summary

This PR addresses issue #3348 by adding optional chaining and a fallback to prevent TypeError when document.body is temporarily null during heavy DOM manipulation. The fix is minimal, defensive, and includes comprehensive test coverage.

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✅ Positives

1. Minimal, Targeted Fix: The change is exactly what's needed - adding ?.scrollLeft and ?.scrollTop with a || 0 fallback. This is defensive programming at its best.

2. Comprehensive Test Coverage: The test file (DocumentProjectionNode.test.ts) is excellent:

   • Tests the null body edge case
   • Tests fallback chain: documentElement → body → 0
   • Tests all branches of the conditional logic
   • Properly cleans up with afterEach hook
   • Good test names that clearly describe behavior

3. Consistent with Codebase Style:

   • Uses proper describe/test structure matching existing tests
   • Follows naming conventions
   • Tests are colocated in __tests__ directory per project structure

4. Safe Fallback Behavior: Returns 0 when both scroll values are unavailable, which is the correct neutral value for scroll positions.

---

🤔 Minor Considerations

1. Optional Chaining + Nullish Coalescing Mix

The current implementation:

x: document.documentElement.scrollLeft || document.body?.scrollLeft || 0

This works correctly but mixes || (logical OR) with ?. (optional chaining). While functional, this could potentially fail if scrollLeft is explicitly 0 (though in practice, || 0 handles that).

Not a bug, just noting that the original issue suggested ?? (nullish coalescing):

x: (document.documentElement.scrollLeft || document.body?.scrollLeft) ?? 0

The current implementation is fine since scrollLeft returns a number, and || correctly handles 0 values in this fallback chain.

2. Test Implementation Detail

The tests directly test the logic expression rather than importing and calling DocumentProjectionNode.measureScroll(). This is acceptable for unit testing the expression, but you might consider:

import { DocumentProjectionNode } from "../DocumentProjectionNode"

test("measureScroll handles null body", () => {
    Object.defineProperty(document, "body", {
        value: null,
        writable: true,
        configurable: true,
    })
    
    expect(() => DocumentProjectionNode.measureScroll()).not.toThrow()
    expect(DocumentProjectionNode.measureScroll()).toEqual({ x: 0, y: 0 })
})

This would test the actual API surface rather than duplicating the implementation logic. However, the current approach is still valid and tests the fix effectively.

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🔍 Code Quality Assessment

• Correctness: ✅ Fix correctly addresses the null reference issue
• Performance: ✅ No performance impact (same number of property accesses, just safer)
• Security: ✅ No security concerns
• Maintainability: ✅ Code is clear and well-tested
• Best Practices: ✅ Follows defensive programming principles

---

📋 Test Coverage

The test suite covers:

• ✅ Null body edge case (primary fix)
• ✅ Zero values fallback
• ✅ documentElement priority
• ✅ Body fallback when documentElement is 0
• ✅ Proper cleanup

Excellent test coverage for this edge case!

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🎯 Recommendation

APPROVE ✅

This is a solid defensive fix for a rare but real edge case. The implementation is correct, the tests are comprehensive, and the change is minimal. The fix appropriately handles the scenario where document.body might be temporarily null during aggressive DOM manipulation.

Great work addressing this edge case! 🚀