Added tests & normal function convertion (#445)

* added factorial calculator recursively

* added tests and converted to normal function

* Added tests & normal function convertion

* Update factorialCalculator.js

updated code "design" not sure why test fails on such arbitrary things.

* Update and rename factorialCalculator.js to factorial.js

Co-authored-by: vinayak <itssvinayak@gmail.com>

Recursive/factorial.js

@@ -0,0 +1,16 @@
+// function to find factorial using recursion
+// example :
+//    5! = 1*2*3*4*5 = 120
+//    2! = 1*2 = 2
+
+const factorial = (n) => {
+  if (n === 0) {
+    return 1
+  }
+  return n * factorial(n - 1)
+}
+
+// testing
+console.log(factorial(4))
+console.log(factorial(15))
+console.log(factorial(0))