const API_BASE = "https://pokeapi.co/api/v2";

/**
 * Fetches JSON data from a given URL.
 * @param {string} url 
 * @returns {Promise<any>}
 */
const fetchJSON = (url) => 
  fetch(url).then((res) => res.ok ? res.json() : Promise.reject(`Error: ${res.status}`));

/**
 * Fetches a Pokémon's species data to get the evolution chain URL.
 * @param {string|number} identifier - Pokémon name or ID.
 * @returns {Promise<string>}
 */
const getEvolutionChainUrl = (identifier) =>
  fetchJSON(`${API_BASE}/pokemon-species/${identifier}`)
    .then((data) => data.evolution_chain.url);

/**
 * Fetches and extracts evolution chain details.
 * @param {string} url - Evolution chain API URL.
 * @returns {Promise<string[]>} - Evolution chain sequence.
 */
const getEvolutionChain = (url) => 
  fetchJSON(url).then((data) => extractEvolutionChain(data.chain));

/**
 * Recursively extracts evolution names from chain data.
 * @param {Object} chain 
 * @returns {string[]}
 */
const extractEvolutionChain = (chain) => {
  const evolutions = [chain.species.name];
  let next = chain.evolves_to;
  
  while (next.length) {
    evolutions.push(next[0].species.name);
    next = next[0].evolves_to;
  }
  
  return evolutions;
};

/**
 * Gets the evolution chain of a given Pokémon by name or ID.
 * @param {string|number} identifier
 * @returns {Promise<void>}
 */
const getPokemonEvolutionChain = (identifier) =>
  getEvolutionChainUrl(identifier)
    .then(getEvolutionChain)
    .then((evolutions) => console.log(`Evolution chain: ${evolutions.join(" → ")}`))
    .catch((error) => console.error(`Failed to fetch evolution chain: ${error}`));

// Test cases
getPokemonEvolutionChain("pikachu"); // Pikachu's evolution chain
getPokemonEvolutionChain(40);        // Pokémon with ID 40