Duplicate JavaScript initialisation and event handlers in Magento
Carousels and widgets that initialise repeatedly attach duplicate event handlers, waste CPU and cause odd behaviour. How to detect and guard against it.
Carousels and widgets that initialise repeatedly attach duplicate event handlers, waste CPU and cause odd behaviour. How to detect and guard against it.
A product-recommendations carousel kept re-initialising Slick and attaching duplicate event handlers. Guarding initialisation removed the extra work and the unpredictable behaviour.
Symptoms
- Carousels jumping or double-sliding
- Clicks firing twice
- Growing memory use on long sessions
- Worse INP on product and category pages
The fix
Check whether the component already exists (for example a Slick-initialised class) before initialising, namespace and remove event handlers before re-binding, and initialise from one place.
Common mistakes to avoid
How we help with performance
Frequently asked questions
How do I find duplicate handlers?
Use Chrome DevTools’ event listener panel and Performance recordings.
Does this affect Core Web Vitals?
It can worsen INP and overall responsiveness.
Is this common?
Very, especially with recommendation carousels and AJAX blocks.