That works because the length field acts as a fill pointer. In Common Lisp, it's also a common technique to empty an array in-place by setting the fill pointer

That works because the length field acts as a fill pointer. In Common Lisp, it's also a common technique to empty an array in-place by setting the fill pointer to 0. The concept is quite old! In most cases, there isn't a beneficial performance boost from emptying an array this way. One needs to allocate in a loop or do something equally inefficient to measure the difference between `coll = []` and `coll.length = 0`. (This assumes the language runtime does not statically allocate an immutable, empty array and use that to represent all empty arrays. Clojure does this, for instance, so no allocations occur when binding a variable to [] or other empty collections).