text: //somone who does this present-tense verb
  //turn 'walk' into 'walker'
  'use strict';
  const actor = function(str) {
    str = str || '';
    const irregulars = {
      'tie': 'tier',
      'dream': 'dreamer',
      'sail': 'sailer',
      'run': 'runner',
      'rub': 'rubber',
      'begin': 'beginner',
      'win': 'winner',
      'claim': 'claimant',
      'deal': 'dealer',
      'spin': 'spinner'
    };
    const dont = {
      'aid': 1,
      'fail': 1,
      'appear': 1,
      'happen': 1,
      'seem': 1,
      'try': 1,
      'say': 1,
      'marry': 1,
      'be': 1,
      'forbid': 1,
      'understand': 1,
      'bet': 1
    };
    const transforms = [{
      'reg': /e$/i,
      'repl': 'er'
    }, {
      'reg': /([aeiou])([mlgp])$/i,
      'repl': '$1$2$2er'
    }, {
      'reg': /([rlf])y$/i,
      'repl': '$1ier'
    }, {
      'reg': /^(.?.[aeiou])t$/i,
      'repl': '$1tter'
    }];
  
    if (dont.hasOwnProperty(str)) {
      return null;
    }
    if (irregulars.hasOwnProperty(str)) {
      return irregulars[str];
    }
    for (let i = 0; i < transforms.length; i++) {
      if (str.match(transforms[i].reg)) {
        return str.replace(transforms[i].reg, transforms[i].repl);
      }
    }
    return str + 'er';
  };
  
  // console.log(verb_to_actor('set'))
  // console.log(verb_to_actor('sweep'))
  // console.log(verb_to_actor('watch'))
  module.exports = actor;
  