답안

//abcdefgh
//20190923
const user_s = 'aacdddddddddfffffffffgghhh';
let result_s = '';

console.log(`${user_s.match(/a/g).length}${Number(user_s.match(/b/g))}${user_s.match(/c/g).length}${user_s.match(/d/g).length}${Number(user_s.match(/e/g))}${user_s.match(/f/g).length}${user_s.match(/g/g).length}${user_s.match(/h/g).length}`);