fix(math): add input validation for negative numbers in factorial

The factorial function silently returned 1 for any negative input,
which is mathematically incorrect since factorial is undefined for
negative integers. Added a guard clause that throws a descriptive
error when a negative number is passed.

Added test cases to verify the error is thrown for negative inputs.

Author: fauzan171

src/algorithms/math/factorial/__test__/factorial.test.js

@@ -8,4 +8,16 @@ describe('factorial', () => {
     expect(factorial(8)).toBe(40320);
     expect(factorial(10)).toBe(3628800);
   });
+
+  it('should throw an error for negative numbers', () => {
+    const negativeFactorial = () => {
+      factorial(-1);
+    };
+    expect(negativeFactorial).toThrow('Factorial is not defined for negative numbers');
+
+    const negativeFactorial2 = () => {
+      factorial(-5);
+    };
+    expect(negativeFactorial2).toThrow('Factorial is not defined for negative numbers');
+  });
 });

src/algorithms/math/factorial/factorial.js

@@ -3,6 +3,11 @@
  * @return {number}
  */
 export default function factorial(number) {
+  // Factorial is not defined for negative numbers.
+  if (number < 0) {
+    throw new Error('Factorial is not defined for negative numbers');
+  }
+
   let result = 1;
 
   for (let i = 2; i <= number; i += 1) {