index.js

const {Dex} = require('@pkmn/dex');
const {Generations} = require('@pkmn/data');
const fs = require('fs');

const nonT5File = fs.readFileSync('non_t5.txt').toString().split('\n');

// These species are unobtainable outside of their own generations, but @pkmn/dex doesn't contain
// the artificial 'natDexTier' field which allows Pokémon Showdown to track this so we harcode it.
// If using @pkmn/sim instead, this list can be replaced with a `d.natDexTier !== 'Illegal'` check.
const NATDEX_UNOBTAINABLE_SPECIES = [
  'Eevee-Starter', 'Floette-Eternal', 'Pichu-Spiky-eared', 'Pikachu-Belle', 'Pikachu-Cosplay',
  'Pikachu-Libre', 'Pikachu-PhD', 'Pikachu-Pop-Star', 'Pikachu-Rock-Star', 'Pikachu-Starter',
  'Eternatus-Eternamax',
];

const NATDEX_EXISTS = (d, g) => {
  // The "National Dex" rules only apply to gen 8+, but this ExistsFn gets called on all generations
  if (g < 8) return Generations.DEFAULT_EXISTS(d, g);
  // These checks remain unchanged from the default existence filter
  if (!d.exists) return false;
  if (d.kind === 'Ability' && d.id === 'noability') return false;
  // "National Dex" rules allows for data from the past, but not other forms of nonstandard-ness
  if ('isNonstandard' in d && d.isNonstandard && d.isNonstandard !== 'Past' && d.isNonstandard !== 'Unobtainable') return false;
  // Filter out the unobtainable species
  if (d.kind === 'Species' && NATDEX_UNOBTAINABLE_SPECIES.includes(d.name)) return false;
  // Nonstandard items other than Z-Crystals and Pokémon-specific items should be filtered
  return !(d.kind === 'Item' && ['Past', 'Unobtainable'].includes(d.isNonstandard) &&
    !d.zMove && !d.itemUser && !d.forcedForme);
};

const gens = new Generations(Dex, NATDEX_EXISTS);
const gen9 = gens.get(9);

let species = [...gen9.species];

for (let x of nonT5File) {
  x = x.trim();
  if (x === '') continue;

  const y = gen9.species.get(x);
  if (!y) throw new Error('couldn\'t get ' + x);

  const index = species.indexOf(y);
  if (index === -1) continue;

  species.splice(index, 1);

  if (!y.formes) continue;
  for (const form of y.formes) {
    const y = gen9.species.get(form);
    if (!y) throw new Error('couldn\'t get ' + form);

    const index = species.indexOf(y);
    if (index === -1) {
      continue;
    }

    species.splice(index, 1);
  }
}

species.sort((a, b) => a.bst - b.bst);

for (let x of species) {
  if (x.forme.endsWith('Gmax') || x.forme === 'Totem') continue;
  console.log(x.name);
}