merge: Add test case (TheAlgorithms#851)

* Add test case

* minor fix

* delete files

* rename file

Author: Yatin-kathuria

Recursive/Factorial.js

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+/**
+ * @function Factorial
+ * @description function to find factorial using recursion.
+ * @param {Integer} n - The input integer
+ * @return {Integer} - Factorial of n.
+ * @see [Factorial](https://en.wikipedia.org/wiki/Factorial)
+ * @example 5! = 1*2*3*4*5 = 120
+ * @example 2! = 1*2 = 2
+ */
+
+const factorial = (n) => {
+  if (n === 0) {
+    return 1
+  }
+  return n * factorial(n - 1)
+}
+
+export { factorial }

Recursive/factorial.js

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+import { factorial } from '../Factorial'
+
+describe('Factorial', () => {
+  it('should return factorial 1 for value "0"', () => {
+    expect(factorial(0)).toBe(1)
+  })
+
+  it('should return factorial 120 for value "5"', () => {
+    expect(factorial(5)).toBe(120)
+  })
+})