diff --git a/calculator/calculator.js b/calculator/calculator.js
index d45cc64..f295a39 100644
--- a/calculator/calculator.js
+++ b/calculator/calculator.js
@@ -20,7 +20,25 @@ const power = function(a, b) {
   return Math.pow(a, b);
 };
 
+/* 
+ * If you want to write a handmade power() function by yourself,
+ * Here's a simple example. 
+ * Only work with integer as input. 
+ */
+const intPower = function(base, exponent) {
+  if (exponent === 0) return 1;
+  if (base === 0) return (exponent > 0)? 0 : Infinity;
+  let result = 1;
+  if (exponent > 0) {
+    while (exponent-- > 0) result *= base;
+  } else {
+    while (exponent++ < 0) result /= base;
+  }
+  return result;
+};
+
 const factorial = function(n) {
+  if (n < 0) return undefined;
   if (n === 0) return 1;
   let product = 1;
   for (let i = n; i > 0; i--) {
@@ -32,6 +50,7 @@ const factorial = function(n) {
 // This is another implementation of Factorial that uses recursion
 // THANKS to @ThirtyThreeB!
 const recursiveFactorial = function(n) {
+  if (n < 0) return undefined;
   if (n === 0) {
     return 1;
   }
diff --git a/calculator/calculator.spec.js b/calculator/calculator.spec.js
index dc317ec..55c04a1 100644
--- a/calculator/calculator.spec.js
+++ b/calculator/calculator.spec.js
@@ -39,22 +39,62 @@ describe('sum', () => {
 });
 
 describe('multiply', () => {
+	test.skip('computes the product of an empty array', () => {
+		expect(calculator.multiply([])).toBe(0);
+	});
 	test.skip('multiplies two numbers', () => {
 		expect(calculator.multiply([2,4])).toBe(8);
 	});
-
 	test.skip('multiplies several numbers', () => {
 		expect(calculator.multiply([2,4,6,8,10,12,14])).toBe(645120);
 	});
 });
 
+/* 
+ * Base facts:
+ * 	=> x to the power of 0 is always 1.
+ * 	=> with base of 0, the exponent must NOT be negative, otherwise the result will be Infinity.
+ */
 describe('power', () => {
 	test.skip('raises one number to the power of another number', () => {
 		expect(calculator.power(4,3)).toBe(64); // 4 to third power is 64
 	});
+	test.skip('to the negative exponent', () => {
+		expect(calculator.power(2,-2)).toBe(0.25);
+	});
+	test.skip('with negative base', () => {
+		expect(calculator.power(-2,2)).toBe(4);
+	});
+	test.skip('with negative base', () => {
+		expect(calculator.power(-2,3)).toBe(-8);
+	});
+	test.skip('negative base to the negative exponent', () => {
+		expect(calculator.power(-2,-2)).toBe(0.25);
+	});
+	test.skip('negative base to the negative exponent', () => {
+		expect(calculator.power(-2,-3)).toBe(-0.125);
+	});
+	test.skip('to the power of 0', () => {
+		expect(calculator.power(2,0)).toBe(1);
+	});
+	test.skip('to the power of 0', () => {
+		expect(calculator.power(-2,0)).toBe(1);
+	});
+	test.skip('to the power of 0', () => {
+		expect(calculator.power(0,0)).toBe(1);
+	});
+	test.skip('with base of 0', () => {
+		expect(calculator.power(0,2)).toBe(0);
+	});
+	test.skip('with base of 0', () => {
+		expect(calculator.power(0,-2)).toBe(Infinity);
+	});
 });
 
 describe('factorial', () => {
+	test.skip('factorial of negative number should be undefined', () => {
+		expect(calculator.factorial(-1)).toBeUndefined();
+	});
 	test.skip('computes the factorial of 0', () => {
 		expect(calculator.factorial(0)).toBe(1); // 0! = 1
 	});