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.