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The clean-up step may seem strange, but it’s a memory-usage optimization. When a language runtime executes a series of function calls it does not need to keep already-exited stack frames around. Branching function calls only form a tree in the time dimension; at any given snapshot in time there exists only a single stack of function calls in memory, tracing the current path through the tree. If we did not set our child frames to null we would end up building the whole tree up in memory and retaining it until the recursive processing is complete. Removing references to them allows the runtime to reclaim their memory at its leisure. If you’ve ever wished that you could deal with manual memory management in JavaScript, now’s your chance!