What causes "Uncaught TypeError: Cannot read properties of undefined"?

It means your code read a property on something that does not exist yet, for example cart.items.length when cart is undefined. On e-commerce stores this is almost always one of three things: (1) the script ran before the DOM element it needs was rendered, (2) an async response (cart, customer data) had not arrived, or (3) a dependency failed to load. We read the stack trace to the exact line, find the null source, and fix the load order or add a proper guard, not a try/catch band-aid.