When a block ends, its lexical environment is eligible for GC (unless a closure keeps it alive). This lets the GC free block-scoped variables sooner than function-scoped var variables, which stay alive until the function returns.

Because var is function-scoped, not block-scoped. A var inside a block leaks to the enclosing function scope and stays alive until the function returns, even if the block has ended. let and const are freed when the block ends.

Remove handlers when no longer needed, do not close over large objects unless necessary, use WeakRef or WeakMap where appropriate, and keep closures short-lived. Avoid accumulating closures that each keep a large object alive.