Best Practices

Key Concept: Prevent Flickering

The most jarring A/B testing issue is the "flash of original content" (FOOC) where users briefly see one variant before it switches to another. This creates a poor user experience and can skew test results.

Implementation Pattern

(() = {
  // Helper functions
  const hideVariant = (variant) = 
    document.querySelectorAll('.variant-b')
      .forEach((el) = el.remove());
  
  const cleanupVariant = () =
    document.querySelectorAll('.variant-a')
      .forEach((el) = el.classList.remove('variant-a'));
  
  document.addEventListener('readystatechange', () = {
    if (document.readyState === 'interactive') {
      hideVariant('b');
      cleanupVariant('a');
    }
  });
})()

Why This Works

• readystatechange fires before any content renders
• interactive state means DOM is ready but before images/styles load
• Removing unwanted variants early prevents any visible switching
• Cleaning up variant classes keeps the DOM tidy