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).
foo.splice(0) is actually sufficient. From the docs:
"If deleteCount is omitted, or if its value is greater than or equal to the number of elements after the position specified by start, then all the elements from start to the end of the array will be deleted."
and just when you think you know it all it will get a major overhaul. Then there will come another language that compiles to JS, and then that language will die out and you have to rewrite your code into the next language that compiles to JS.
