Modular arithmetic for JavaScript. See docs.
import { Montgomery } from '@arithmetic-operations-for/integers-modulo-n-big-endian' ;
const int = s => s.split('').map(x => +x) ;
const str = x => x.join('') ;
const base = 10 ;
const N = int('57896044618658097711785492504343953926634992332820282019728792003956564819949') ;
const mont = new Montgomery(base, N) ;
const a = int('57896044618658097711785492504343953926634992332820281301830804312103976049700') ;
const b = int('57896044618658097711785492504343953926634992332820282019728792003955491078125') ;
const _a = mont.from(a) ;
const _x = mont.pow(_a, b) ; // a^b % N
const x = mont.out(_x) ;
str(x) ; // 40504055762004792620159537441437949886475081163592261781667958256380085618313
const c = int('717897987691852588770249') ;
const _y = mont.pow(_a, c, false) ; // a^-c % N
const y = mont.out(_y) ;
str(y) ; // 51815386826945512755518685488363592823806772017922768894709047770322605499358