The usual approach of counting the number of uppercase and lower letters in a string is done by using a counter and a loop:

var upperCount = 0;
var lowerCount = 0;
var str = "AbCdEfGhiJkL";
for(var i=0; i<str.length; i++){
    if(str[i] == str[i].toUpperCase()){

That's nice but here's shorter and faster implementation:

var str = "AbCdEfGhiJkL";
var lowerCount = str.length - str.replace(/[A-Z]/g, '').length;
var upperCount = str.length - str.replace(/[a-z]/g, '').length;

The lowerCount variable is taking the difference of the lengths between the original string and the string with lowercase letters only because the .replace() method replaced the pattern of uppercase letters [A-Z] with empty spaces. The upperCount variable does the opposite of what the lowerCount variable does.

Hope you found this trick useful!