помощь в решении задачи на JS

Вы можете использовать условный оператор ?.

function likes(names) {
  len = names.length;
  
  return (len === 0) ? `no one likes this` : 
    (len === 1) ? `${names[0]} likes this` :
    (len === 2) ? `${names[0]} and ${names[1]} like this` :
    (len === 3) ? `${names[0]}, ${names[1]} and ${names[2]} like this` : 
    `${names[0]}, ${names[1]} and ${len - 2} others like this`;
}