Add solution directories for exercises

08_calculator/solution/calculator-solution.js

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+const add = function (a, b) {
+    return a + b;
+};
+
+const subtract = function (a, b) {
+    return a - b;
+};
+
+const sum = function (array) {
+    return array.reduce((total, current) => total + current, 0);
+};
+
+const multiply = function (array) {
+    return array.length
+        ? array.reduce((accumulator, nextItem) => accumulator * nextItem)
+        : 0;
+};
+
+const power = function (a, b) {
+    return Math.pow(a, b);
+};
+
+const factorial = function (n) {
+    if (n === 0) return 1;
+    let product = 1;
+    for (let i = n; i > 0; i--) {
+        product *= i;
+    }
+    return product;
+};
+
+// This is another implementation of Factorial that uses recursion
+// THANKS to @ThirtyThreeB!
+const recursiveFactorial = function (n) {
+    if (n === 0) {
+        return 1;
+    }
+    return n * recursiveFactorial(n - 1);
+};
+
+module.exports = {
+    add,
+    subtract,
+    sum,
+    multiply,
+    power,
+    factorial,
+};