revertWordInString.js

function revertWordInString(str) {
    if (str.trim().length === 0) {
      throw 'err';
    }
  
    return str
      .split(/\s/)
      .map(str => str.length 
          ? str.split('').reverse().join('')
          : str)
      .join(' ');
}

console.log(revertWordInString('one two three  four       five'));