Это еще один способ:
'/FOLDER1/FOLDER2/FOLDER3/'.replace(/(\w*\/)$/, '');
//gives you: "/FOLDER1/FOLDER2/"
еще один:
var s = "FOLDER1/FOLDER2/FOLDER3/";
var ss = s.replace(/\/$/, '');
ss.substring(0, ss.lastIndexOf('/'));
другой:
const [path1, path2] = "FOLDER1/FOLDER2/FOLDER3/".split("/")
console.log('path: ', `/${path1}/${path2}/`);
другой:
var s = "FOLDER1/FOLDER2/FOLDER3/";
const [newPath] = s.split(/(\w+\/)$/)
//"FOLDER1/FOLDER2/"