feat(guide): Add a new solution using reduce to "Factorialize a Number"

guide/english/certifications/javascript-algorithms-and-data-structures/basic-algorithm-scripting/factorialize-a-number/index.md

@@ -144,3 +144,28 @@ factorialize(5);
 
 *   <a href='https://www.geeksforgeeks.org/tail-recursion/' target='_blank' rel='nofollow'>Tail Recursion</a>
 </details>
+
+<details><summary>Solution 4 (Click to Show/Hide)</summary>
+
+```js
+function factorialize(num, factorial = 1) {
+  return num < 0 ? 1 : (
+    new Array(num).fill(undefined)
+                  .reduce((product,val, index) => product * (index + 1), 1)
+  );
+}
+
+factorialize(5);
+```
+
+#### Code Explanation
+* In this solution, we used "reduce" function to find the factorial value of the number. This is an intermadiate example.
+
+* We have created an array which has length `num`. And we filled all elements of the array as `undefined`. In this case, we have to do this because empty arrays couldn't reducible. You can fill the array as your wish by the way. This depends on your engineering sight completely.
+
+* In `reduce` function's accumulator is calling `product` this is also our final value. We are multiplying our index value with the product to find `factorial` value.
+
+* We're setting product's initial value to 1 because if it was zero products gets zero always.
+
+* Also the factorial value can't calculate for negative numbers, first of all, we're testing this.
+</details>