sortStringAndIsSubsequence.js

var isSubsequence = function(str, sub) {
	currentIndex = 0;
	for (var i = 0; i<str.length; i++) {
		if (str[i] === sub[currentIndex]) {
			currentIndex +=1;
		} 
	}
	return currentIndex >= sub.length ? true : false;
}

var sortStringByFrequency = function(str) {
	var store = {}
	var sortable = []
	var result = []
	for (var i = 0; i<str.length; i++) {
		var letter = str[i];
		if (letter in store) {
			store[letter] += 1;
		} else {
			store[letter] = 1;
		}
	}
	for (letter in store) {
		sortable.push([letter, store[letter]])
	}

	sortable.sort(function(a, b) {
		return b[1] - a[1];
	})

	sortable.forEach(function(arr) {
		for (var i = 0; i<arr[1]; i++) {
			result.push(arr[0]);
		}
	})

	return result.join('');

}