There are going to be quite a few ways to achieve your goal with JavaScript. A simple solution could be something like this:
let myURL = "SampleProject/profile/aA12";
let result = myURL.split('/').pop();
// returns "aA12"
The .split('/') method is dividing your string up into an array using the / character, and .pop() is simply returning the last element of that array.
Hope this helps! If you were looking for more advanced matching, i.e. if you wanted to ignore a potential query string on the end of the URL parameter, you could use regular expressions.