decipherThis.js

function decipherThis(str) {
  //have fun!
  // get a regex to match all the "numbers" 
  const rgx = /\d+/g;
  // get an array for rgx matches
  const matches = str.match(rgx);
  // loop on each match to replace with the String.fromCharCode to get the character
  matches.forEach((element) => {
    str = str.replace(element, String.fromCharCode(element));
  });

  // convert the str to arr
  str = str.split(' ')
  // a new arr to catch all the word once deciphered
  const newStr = []

  // loop on each word to decipher it
  for (let i = 0; i < str.length; i++){
    if (str[i].length < 3) {
      newStr.push(str[i]);
    }else{
      // extract the characters needed with the substring method
      const fw = str[i].substring(0,1);
      const lmodw = str[i].substring(str[i].length-1);
      const restmodw = (str[i].length > 2) ? str[i].substring(2,str[i].length-1) : '';
      const smod = (str[i].length > 2) ? str[i].substring(2,1) : '';

      // get all the extractions together and push it to the newStr
      const all = fw+lmodw+restmodw+smod;
      newStr.push(all);
    };
  }

  // .join() the [newStr] to make it a str
  return newStr.join(' ')
}

console.log(
  decipherThis("72eva 97 103o 97t 116sih 97dn 115ee 104wo 121uo 100o")
);