Fast solution

var shufflin = function (n, c) {
	var deck;
	var count = 1;
	//array to hold the frequency of the cards appearing in the same index
	var frequency = new Array(n);

	//creates a deck with n values
	var createDeck = function (n) {
		var arr = [];
		for (var i = 0; i< n; i++) {
			arr.push(i);
		}
		return arr;
	};

	deck = createDeck(n);


	var checkFrequency = function (arr) {
		for (var i = 0; i < arr.length; i++) {
			frequency[i] = frequency[i] === undefined && arr[i] === i ? count : frequency[i] || undefined;
		}
	};
  
  //greatest common divisor
  var gcd = function (x, y) {
		while (y !== 0) {
			var z = x % y;
			x = y;
			y = z;
		}
		return x;
	};

  //least common multiple
	var  lcm = function (x, y){
    return (x * y / gcd(x, y));
	};

  //Recursively iterate through pairs of arguments in the frequency array
	var lcmR = _.memoize(function (args){
		if(args.length == 2){
			return lcm(args[0], args[1]);
		} else {
			var arg0 = args[0];
			args.shift();
			return lcm(arg0, lcmR(args));
		}
	});

	var shuffle = function (deck, c) {
		var s1 = deck.slice(n-c);//upper slice
		var s2 = deck.slice(0, n-c);//lower slice
		var len = c < n-c ? c : n-c; 
		var nDeck = [];

		//perfect shuffle
		for (var i = 0; i < len*2; i+=2) {
			nDeck[i] = s1.shift();
			nDeck[i+1] = s2.shift();
		};

		var rest = s2.length ? s2 : s1;
		//adds the rest of the cards on top
		nDeck = nDeck.concat(rest);
		checkFrequency(nDeck);
		if (_.contains(frequency, undefined)) { 
			count++;
			shuffle(nDeck, c);
		}else{
			var nums = _.uniq(frequency);
			count = lcmR(nums);
			console.log(count);
			return count;
		}
	};
	shuffle(deck,c);
};

Naive solution

//compares the arrays
var arraysEqual = function (arr1, arr2) {
    if(arr1.length !== arr2.length)
        return false;
    for(var i = arr1.length; i--;) {
        if(arr1[i] !== arr2[i])
            return false;
    }

    return true;
};

var deck = function (n) {
	var arr = [];
	for (var i = 0; i< n; i++) {
		arr.push(i);
	}
	return arr;
}

var count = 1;
var shufflin = function (deck, c) {
	var n = deck.length;
	var s1 = deck.slice(n-c);
	var s2 = deck.slice(0, n-c);
	var len = c < n-c ? c : n-c; 

	var nDeck = [];
	for (var i = 0; i < len*2; i+=2) {
		nDeck[i] = s1.shift();
		nDeck[i+1] = s2.shift();
	}
	var rest = s2.length ? s2 : s1;


	nDeck = nDeck.concat(rest);
	if(arraysEqual(nDeck, nDeck.slice().sort(function(a,b) {return a-b}))){
		console.log(nDeck);
		console.log(count);
		return;
	} else {
		count++;
		console.log(nDeck);
		shufflin(nDeck, c);
	}
}