var arr = ["a","b","a","aa","a","c","b"];
var tmp = {};
arr.forEach(function(str){
  tmp[str] = ~~tmp[str] + 1;
});
var res = [];
for(var i in tmp){
  res.push({name: i, num: tmp[i]});
}
res.sort(function(obja, objb){
	return objb.num - obja.num;
})
document.write(JSON.stringify(res));