Understanding and Solving window.innerWidth Issues in Mobile A/B Testing
Avoid incorrect device detection in A/B testing! Learn how window.innerWidth can cause mobile targeting issues and how to fix them effectively.
🚀 IN THIS ARTICLE YOU WILL:
- Understand the Challenge with Mobile Device Detection
- Learn Why This Happens
- See the Impact on A/B Testing
- Discover Solutions and Best Practices
- Follow Best Practices for Device Targeting
- Avoid Common Pitfalls in Mobile A/B Testing
📱 The Challenge with Mobile Device Detection
When running A/B tests that rely on window.innerWidth
for device targeting, you may encounter an unexpected issue where mobile devices initially report desktop-width values. This can create significant problems, especially for experiments that depend on accurate device detection at page load.
❓ Why Does This Happen?
This issue stems from how mobile browsers handle viewport initialization:
- When a page first loads, the browser starts with a wider default viewport (typically around 980px).
- Shortly after, the viewport adjusts to the actual device width (e.g., 390px for many modern smartphones).
- This adjustment isn't instantaneous, leading to a race condition where your A/B test code may execute before the correct viewport width is set.
🔍 Impact on A/B Testing
This behavior can negatively affect your experiments in several ways:
✅ Mobile-specific variations may fail to trigger properly.
✅ Device-dependent logic might execute incorrectly.
✅ The user experience could be compromised during the initial page load.
✅ Solutions and Best Practices
To mitigate this issue, consider the following approaches:
1. Implement a Waiting Function
Introduce a short delay to ensure the viewport has settled before executing your A/B test logic.
const windowInterval = setInterval(() => {
const isMobile = window.innerWidth < 768;
if (isMobile) {
// Your mobile-specific code here
}
setTimeout(() => clearInterval(windowInterval), 4000); // Safety timeout
}, 100);
2. Use Multiple Detection Methods
Combine window.innerWidth
, media queries, and user-agent detection for improved accuracy.
const isMobileDevice = () => {
return (
/Android|webOS|iPhone|iPad|iPod|BlackBerry|IEMobile|Opera Mini/i.test(navigator.userAgent) &&
window.matchMedia("(max-width: 768px)").matches
);
};
3. Listen for Viewport Changes
Since viewport adjustments happen after the initial page load, use the resize
event to detect changes dynamically.
window.addEventListener('resize', () => {
const isMobile = window.innerWidth < 768;
// Update your variation accordingly
});
4. Use the DOMContentLoaded
Event
Ensure your detection logic runs after the DOM is fully loaded.
document.addEventListener('DOMContentLoaded', () => {
const checkDeviceWidth = () => {
const isMobile = window.innerWidth < 768;
// Your device-specific code here
};
checkDeviceWidth();
});
🎯 Best Practices for Device Targeting
🔹 Test on Real Devices – Don't rely solely on browser dev tools for mobile testing.
🔹 Use Multiple Indicators – Combine window.innerWidth
, media queries, and user agent detection.
🔹 Implement Fallbacks – Ensure a backup logic is in place if the primary detection method fails.
🔹 Consider Performance – Balance polling intervals with performance requirements.
🔹 Test Different Scenarios – Verify behavior on page load, orientation change, and resize events.
QA Recommendations
When testing mobile variations:
✅ Use Force Variation links instead of Preview links.
✅ Test across multiple device types and browsers.
✅ Check functionality immediately after page load.
✅ Verify behavior during orientation changes.
✅ Monitor performance impact of detection methods.
Common Pitfalls to Avoid
🚫 Relying solely on window.innerWidth
at page load.
🚫 Not accounting for orientation changes.
🚫 Missing viewport meta tags.
🚫 Ignoring browser differences in viewport handling.
🚫 Using excessive polling intervals, which can harm performance.