Recursion Function (재귀함수)

Recursion is a function calling itself (함수 스스로를 실행시키는것)

termination - base - recursion 으로 구성됨

function factorial(x){
  if(x<0) return;   // return undefined, termination condition
  if(x===0) return 1;  // return true (1), base case which terminate function with true
  return x * factorial(x-1);  // recursion
}
console.log(factorial(6));  // 720

function revStr(str){
  if(str === '') return '';
  return revStr(str.substr(1)) + str[0];
}
console.log(revStr('world'));  // dlrow