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.