Marker.js updateMarker logic will re-render if new position has the same coordinates (LAT + LONG) as previous positon

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Summary

The Marker.js updateMarker logic triggers re-renders even when the new position has the same coordinates (LAT + LONG) as the previous position. Despite memoizing markers using useMemo, the entire set of markers still flickers during updates.

Root Cause

  • The default comparison logic in updateMarker checks for strict equality (prevPosition === newPosition), which fails when objects are different instances but have identical coordinates.
  • Memoization (useMemo) does not prevent re-renders if the underlying dependency (position object) changes, even if coordinates remain the same.

Why This Happens in Real Systems

  • Object Reference Inequality: In JavaScript, objects with identical properties are not considered equal if they are different instances.
  • Overly Strict Comparison: The default comparison logic does not account for semantic equality (same coordinates but different objects).
  • React Re-render Behavior: React re-renders components when dependencies change, regardless of memoization if the comparison logic is flawed.

Real-World Impact

  • Performance Degradation: Unnecessary re-renders cause janky UI and increased CPU/GPU usage.
  • User Experience: Marker flickering leads to a poor mapping experience, especially in applications with many markers.
  • Resource Waste: Excessive re-renders consume unnecessary memory and processing power.

Example or Code (if necessary and relevant)

// Current flawed comparison logic
if (prevPosition === newPosition) {
  return; // No update needed
}

// Proposed solution with custom isSame function
if (isSame(prevPosition, newPosition)) {
  return; // No update needed
}

// Example isSame function
function isSame(pos1, pos2) {
  return pos1.lat === pos2.lat && pos1.long === pos2.long;
}

How Senior Engineers Fix It

  • Custom Equality Function: Introduce a user-defined isSame function to compare positions based on semantic equality (coordinates) rather than object reference.
  • Immutable Data Structures: Use immutable position objects to ensure consistent references when coordinates do not change.
  • Debouncing Updates: Implement debouncing for position updates to reduce unnecessary re-renders.

Why Juniors Miss It

  • Assumption of Strict Equality: Juniors often assume === is sufficient for comparison, overlooking object reference issues.
  • Lack of Memoization Understanding: Misunderstanding how useMemo works with changing dependencies leads to ineffective optimizations.
  • Overlooking Semantic Equality: Failure to recognize the need for custom comparison logic based on specific use cases.

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