Unique number allocator maintains number intervals that are not in used. For an example, users can allocate an interval from 1 to 100. And if the user uses 6, the interval will split into two: 1 to 5 and 7 to 100. Unique number allocators will tell the users how many intervals we have right now. And what number is still vacant to use.

Node.js is an event-based server-side JavaScript engine.