What is a Memory Leak?

A memory leak happens when memory that is no longer needed is still referenced, so the garbage collector cannot free it. In React this usually means a component is gone (unmounted) but something it started is still running: a timer, an event listener, a subscription or a request.

• The app gets slower over time.
• Memory usage keeps growing (DevTools → Memory).
• Old callbacks still run and update state that no longer exists.