In JavaScript where there's often a single thread running a long-lived process I think the common case is when you keep allocating stuff through event handlers

In JavaScript where there's often a single thread running a long-lived process I think the common case is when you keep allocating stuff through event handlers and not dereferencing them. A single wasteful build-up of memory is not really what I see discussed when people are discussing memory leaks in JS. As another commenter said, the classic case being adding UI event handlers and then removing the DOM object without dereferencing the handler. Doing this over and over will eventually make the app unresponsive but not immediately.