index.js

function UnionFind(n, key){

	if(! (this instanceof UnionFind))
		return new UnionFind(n);

	if(key && typeof key != 'function'){
		throw new Error("key has to be a function or else left undefined");
	}

	var cnt, id, sz, length;
	key = key || function(a){return a;};
	

	cnt = length = n;
	id = new Array(n);
	sz = new Array(n);
	for (var i = 0; i < n; i++) {
		id[i] = i;
		sz[i] = 1;
	}

	this.size = function(){
		return length;
	};

	this.count = function(){
		return cnt;
	};

	this.find = function (p) {
		p = key(p);
		while(p != id[p]){
			id[p] = id[id[p]];
			p = id[p];
		}

		return p;
	};

	this.connected = function(p, q){
		p = key(p);
		q = key(q);
		ensureIndexWithinBounds(p, q);
		return this.find(p) === this.find(q);
	};

	this.union = function(p, q){
		p = key(p);
		q = key(q);
		ensureIndexWithinBounds(p, q);
		var i = this.find(p)
		,	j = this.find(q);

		if (i === j) return;

		if (sz[i] < sz[j]) 	{id[i] = j; sz[j] += sz[i];}
		else				{id[j] = i; sz[i] += sz[j];}

		cnt--;
		return;
	};

	function ensureIndexWithinBounds(args){
		for (var i = arguments.length - 1; i >= 0; i--) {
			var p = arguments[i];
			if(p >= length)
				throw new Error("Index out of bounds. The maximum index can be length-1");
		}
	}
}

module.exports = UnionFind;