Finding cycles by iterating a list at different speeds until the same value is reached

tortoiseandthehare.js

const EMPTY = null;
const isEmpty = (node) => node === EMPTY;

const pair = (first, rest = EMPTY) => ({first, rest});

const list = (...elements) => {
  const [first, ...rest] = elements;
  return elements.length === 0
    ? EMPTY
    : pair(first, list(...rest))
}

const forceAppend = (list1, list2) => {
  if(isEmpty(list1)) {
    return 'FAIL!'
  }
  if(isEmpty(list1.rest)) {
    list1.rest = list2;
  }
  else {
    forceAppend(list1.rest, list2);
  }
}

const tortoiseAndHare = (aPair) => {
  let tortoisePair = aPair,
      harePair = aPair.rest;
  
  while(true){
    if(isEmpty(tortoisePair) || 
      isEmpty(harePair)) {
      return false;
    }
    if(tortoisePair.first === harePair.first){
      return true;
    }
    
    harePair = harePair.rest;
    
    if(isEmpty(harePair)) {
      return false;
    }
    
    if(tortoisePair.first === harePair.first){
      return true;
    }
    
    tortoisePair = tortoisePair.rest;
    harePair = harePair.rest;
  }
};

const aList = list(1,2,3,4,5);

console.log(aList);
console.log(tortoiseAndHare(aList));