let obj = {a: 24, b: 12, c:21, d:15};



// Get an array of the keys:

let keys = Object.keys(obj);



// Then sort by using the keys to lookup the values in the original object:

keys.sort(function(a, b) { return obj[a] - obj[b] });



console.log(keys);

Note that the above could be done in one line if desired with Object.keys(obj).sort(...). The simple .sort() comparator function shown will work for numeric values only (swap a and b to sort in the opposite direction), but since that's what's in the question I assume it's fine...