a*b parser with first and second

asbFS.js

const assert = require('assert');

/**
 * @param {string} str
 */
const asb = ([fst, scd, ...tail]) => {
  if (fst === 'a' && scd === 'a') return asb([fst, ...tail]);
  if (fst === 'b' && !scd) return true;
  if (fst === 'a' && scd === 'b') return true;

  return false;
};

assert(asb('b') === true);
assert(asb('ab') === true);
assert(asb('aab') === true);
assert(asb('aaab') === true);
assert(asb('aaaab') === true);
assert(asb('aaaaab') === true);
assert(asb('a') === false);
assert(asb('c') === false);
assert(asb('ba') === false);
assert(asb('bb') === false);
assert(asb('bc') === false);
assert(asb('cb') === false);