SOURCE

var temp = 1
function test(num){
   while(num){
       temp = temp*num
       num--
   }
  return temp
}
console.log(test(5))

function test2(num){
    if(num<=1){
          return 1
    }
    else{
        return num*test2(num-1)
    }
}
console.log(test2(5))