Question

https://github.com/joyent/node/blob/master/lib/freelist.js

Making a destroy method is very dangerous. I suggest that if you’re
concerned about this then you use require(‘freelist’) and try to
reclaim memory after using it – which is what I do in net.js

It is for example used in https://github.com/joyent/node/blob/master/lib/http.js, but I still do not completely get it. An example of how this module could be used would be sweet.

Answer 1

It’s very simple way to manage reuse of often created and destroyed objects. The freelist only creates new objects when no currently unused objects are available, reducing memory footprint without having to wait for garbage collection, etc. Using it involves three simple steps:

Creating a new freelist for vectors (imaginary object type you want to reuse):

// Some type of object I want to be able to reuse
var Vector = require('./Vector');
// The freelist itself
var FreeList = require('freelist').FreeList;

// Arguments are (freelistName, maxSize, constructorForNewObjects)
var vectorsList = new FreeList('vectors', 1000, function() {
  // quick way to use constructor with arguments object
  return Vector.apply(Object.create(Vector.prototype), arguments);
});

Getting a new vector from the freelist:

var vector = vectorsList.alloc(/* optional arguments for the Vector constructor /*);

Freeing a vector that’s no longer needed:

vectorsList.free(vector);

That’s it.

One very important thing to note:
Although I have written above to pass the arguments from alloc calls on to the vector constructor, this only happens if there’s no free vector available in the list. Otherwise you just get the first available vector, still constructed with whatever parameters it had at construction time.

If you want to actually initialize every object cleanly, you should probably not put the initialization of the object into the constructor, but into an init function that you always call after getting an object from the freelist.

Ofcourse this could probably be done by the freelist with some clever hacking with calling the constructor again for initialization.