Discussion on: Recursion, or How I Learned to Stop Thinking and Love the Thoughts

[peerreynders]

1. Study the original function.
   

   function factorial(n) {
     return n > 1 ? n * factorial(n - 1) : 1;
   }
   
   console.log(factorial(5)); // 120
   

2. Convert recursive calls into tail calls
   

   function factorial(n, result = 1) {
     return n > 1 ? factorial(n - 1, result * n) : result;
   }
   
   console.log(factorial(5)); // 120
   

3. Introduce a one-shot loop around the function body.
   

   function factorial(n, result = 1) {
     while (true) {
       return n > 1 ? factorial(n - 1, result * n) : result;
       break;
     }
   }
   
   console.log(factorial(5)); // 120
   

4. Replace all recursive tail calls f(x=x1, y=y1, ...) with [x, y, ...] = [x1, y1, ...]; continue.
   

   function factorial(n, result = 1) {
     while (true) {
       if (!(n > 1)) return result;
       [n, result] = [n - 1, result * n];
       continue;
       break;
     }
   }
   
   console.log(factorial(5)); // 120
   

5. Tidy up
   

   function factorial(n) {
     let result = 1;
     while (n > 1)
        [result, n] = [result * n, n - 1];
   
     return result;
   }
   
   console.log(factorial(5)); // 120
   

6. Make more idiomatic
   

   function factorial(n) {
     let result = 1;
     for (; n > 1; result *= n, n -= 1);
   
     return result;
   }
   
   console.log(factorial(5)); // 120
   

If you like that rabbit hole take a look at recursion and Recursion? We don't need no stinking recursion!.