function feibo(a){
  
 if(!a || a <= 0){
   throw new Error("参数错误，必须大于0");
 }else if(a == 1){
  return 1;
 }else if(a == 2){
  return 2;
 }else{
  return feibo(a-1) + feibo(a-2);
 }
}
console.log(feibo(7));<br><br><br><br>

function feibo(n){
  if(n <= 2){
   return 1;
  }
  var s1 = 1;
  var s2 = 1;
  for(var i = 3; i <= n; i++){
   s2 = s2 + s1;
   s1 = s2 - s1;
  }
  return s2;
 }
 console.log(feibo(6));