combination.js

Array.prototype.sliceByIndex = function(idxs){
	var items = this;
	var slicedVals = idxs.map(function(idx){
		return items[idx];
	});
	return slicedVals;
}

var range = function(min,max){
	var nums = [];
	for (var num = min; num <= max; num++){
		nums.push(num);
	}
	return nums;
}

Array.prototype.combination = function(n){
	var items = this;
	var length = items.length;

	var idxs = range(0,length-1);
	var idxPerms = idxs.map(function(idx){
		return [idx];
	});

	for (var idxsIdx = 0; idxsIdx < n-1; idxsIdx++){
		var nextIdxPerms = [];
		idxPerms.forEach(function(idxPerm){
			var lastIdx = idxPerm[idxsIdx];
			var nextIdxs = idxs.filter(function(idx){
				return lastIdx < idx;
			});
			nextIdxs.forEach(function(nextIdx){
					var nextIdxPerm = idxPerm.slice(0);
					nextIdxPerm.push(nextIdx);
					nextIdxPerms.push(nextIdxPerm);
			});
		});
		idxPerms = nextIdxPerms.slice(0);
	}

	perms = idxPerms.map(function(idxPerm){
		return items.sliceByIndex(idxPerm);
	});
	return perms;
}

Array.prototype.repeatedCombination = function(n){
	var items = this;
	var length = items.length;

	var idxs = range(0,length-1);
	var idxPerms = idxs.map(function(idx){
		return [idx];
	});

	for (var idxsIdx = 0; idxsIdx < n-1; idxsIdx++){
		var nextIdxPerms = [];
		idxPerms.forEach(function(idxPerm){
			var lastIdx = idxPerm[idxsIdx];
			var nextIdxs = idxs.filter(function(idx){
				return lastIdx <= idx;
			});
			nextIdxs.forEach(function(nextIdx){
					var nextIdxPerm = idxPerm.slice(0);
					nextIdxPerm.push(nextIdx);
					nextIdxPerms.push(nextIdxPerm);
			});
		});
		idxPerms = nextIdxPerms.slice(0);
	}

	perms = idxPerms.map(function(idxPerm){
		return items.sliceByIndex(idxPerm);
	});
	return perms;
}


var ary = ["a","b","c","d"];

var combs = ary.combination(3);
console.log(combs);
// =>[ [ 'a', 'b', 'c' ],
//     [ 'a', 'b', 'd' ],
//     [ 'a', 'c', 'd' ],
//     [ 'b', 'c', 'd' ] ]

var repeatedCombs = ary.repeatedCombination(3);
console.log(repeatedCombs);
// =>[ [ 'a', 'a', 'a' ],  [ 'a', 'a', 'b' ],  [ 'a', 'a', 'c' ],  [ 'a', 'a', 'd' ],
//     [ 'a', 'b', 'b' ],  [ 'a', 'b', 'c' ],  [ 'a', 'b', 'd' ],  [ 'a', 'c', 'c' ],
//     [ 'a', 'c', 'd' ],  [ 'a', 'd', 'd' ],  [ 'b', 'b', 'b' ],  [ 'b', 'b', 'c' ],
//     [ 'b', 'b', 'd' ],  [ 'b', 'c', 'c' ],  [ 'b', 'c', 'd' ],  [ 'b', 'd', 'd' ],
//     [ 'c', 'c', 'c' ],  [ 'c', 'c', 'd' ],  [ 'c', 'd', 'd' ],  [ 'd', 'd', 'd' ] ]