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,
+};

08_calculator/solution/calculator-solution.spec.js

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+const calculator = require('./calculator-solution');
+
+describe('add', () => {
+    test('adds 0 and 0', () => {
+        expect(calculator.add(0, 0)).toBe(0);
+    });
+
+    test.skip('adds 2 and 2', () => {
+        expect(calculator.add(2, 2)).toBe(4);
+    });
+
+    test.skip('adds positive numbers', () => {
+        expect(calculator.add(2, 6)).toBe(8);
+    });
+});
+
+describe('subtract', () => {
+    test.skip('subtracts numbers', () => {
+        expect(calculator.subtract(10, 4)).toBe(6);
+    });
+});
+
+describe('sum', () => {
+    test.skip('computes the sum of an empty array', () => {
+        expect(calculator.sum([])).toBe(0);
+    });
+
+    test.skip('computes the sum of an array of one number', () => {
+        expect(calculator.sum([7])).toBe(7);
+    });
+
+    test.skip('computes the sum of an array of two numbers', () => {
+        expect(calculator.sum([7, 11])).toBe(18);
+    });
+
+    test.skip('computes the sum of an array of many numbers', () => {
+        expect(calculator.sum([1, 3, 5, 7, 9])).toBe(25);
+    });
+});
+
+describe('multiply', () => {
+    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);
+    });
+});
+
+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
+    });
+});
+
+describe('factorial', () => {
+    test.skip('computes the factorial of 0', () => {
+        expect(calculator.factorial(0)).toBe(1); // 0! = 1
+    });
+
+    test.skip('computes the factorial of 1', () => {
+        expect(calculator.factorial(1)).toBe(1);
+    });
+
+    test.skip('computes the factorial of 2', () => {
+        expect(calculator.factorial(2)).toBe(2);
+    });
+
+    test.skip('computes the factorial of 5', () => {
+        expect(calculator.factorial(5)).toBe(120);
+    });
+
+    test.skip('computes the factorial of 10', () => {
+        expect(calculator.factorial(10)).toBe(3628800);
+    });
+});