word_count.js

function wordCount(str) {
  if (typeof str !== 'string' || !/\w+/.test(str)) return 'Error: bad input';
  
  var words = str.toLowerCase().match(/(\w+(?<!'\w+)'\w+|\w+)/g);

  var incrementWordCount = (word_count, word) => { 
     word_count[word] = 1 + (word_count[word] | 0)
     return word_count;
  };

  return words.reduce(incrementWordCount, {});
}