remove timer and simon

calculator/calculator.js

@@ -1,32 +1,37 @@
-function add () {
-	
+function add(a, b) {
+  return a + b;
 }
 
-function subtract () {
-	
+function subtract(a, b) {
+  return a - b;
 }
 
-function sum () {
-	
+function sum(array) {
+  return array.reduce((current, total) => total + current, 0);
 }
 
-function multiply () {
-	
+function multiply(array) {
+  return array.reduce((current, total) => total * current, 1);
 }
 
-function power() {
-	
+function power(a, b) {
+  return Math.pow(a, b);
 }
 
-function factorial() {
-	
+function factorial(n) {
+  if (n == 0) return 0;
+  let product = 1;
+  for (let i = n; i > 0; i--) {
+    product *= i;
+  }
+  return product;
 }
 
 module.exports = {
-	add,
-	subtract,
-	sum,
-	multiply,
-    power,
-	factorial
-}
+  add,
+  subtract,
+  sum,
+  multiply,
+  power,
+  factorial
+};