Unique number allocator for JavaScript

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