feat(factorial): create exercise

13_factorial/README.md

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+# Exercise 13 - Factorial
+
+Write a recursive [factorial](https://simple.wikipedia.org/wiki/Factorial) function that takes a non-negative integer, and returns the product of all positive integers less than or equal to the input integer. An input of `0` should return `1`. The function should only accept numbers, so `'4'` should not be accepted as it is a string. All invalid inputs should return `undefined`.
+
+For example:
+
+```javascript
+factorial(5); // 5 * 4 * 3 * 2 * 1, Output: 120
+factorial(0); // Output: 1
+factorial(7.2); // Output: undefined
+factorial('4'); // Output: undefined
+```

13_factorial/factorial.js

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+const factorial = function() {
+
+};
+
+// Do not edit below this line
+module.exports = factorial;

13_factorial/factorial.spec.js

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+const factorial = require("./factorial");
+
+describe('factorial', () => {
+  test('4th factorial number is 24', () => {
+    expect(factorial(4)).toBe(24);
+  });
+  test.skip('6th factorial number is 720', () => {
+    expect(factorial(6)).toBe(720);
+  });
+  test.skip('10th factorial number is 3628800', () => {
+    expect(factorial(10)).toBe(3628800);
+  });
+  test.skip('15th factorial number is 1307674368000', () => {
+    expect(factorial(15)).toBe(1307674368000);
+  });
+  test.skip('25th factorial number is 1.5511210043330986e+25', () => {
+    expect(factorial(25)).toBe(1.5511210043330986e+25);
+  });
+  test.skip('0th factorial number is 1', () => {
+    expect(factorial(0)).toBe(1);
+  });
+  test.skip("doesn't accept negatives", () => {
+    expect(factorial(-25)).toBe(undefined);
+  });
+  test.skip("doesn't accept floats", () => {
+    expect(factorial(5.4)).toBe(undefined);
+  });
+  test.skip("doesn't accept a number as a string", () => {
+    expect(factorial('5')).toBe(undefined);
+  });
+  test.skip("doesn't accept strings", () => {
+    expect(factorial('foo')).toBe(undefined);
+  });
+  test.skip("doesn't accept arrays", () => {
+    expect(factorial([5])).toBe(undefined);
+  });
+});

13_factorial/solution/factorial-solution.js

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+const factorial = function(n) {
+  if (!Number.isInteger(n) || n < 0) return;
+  if (n === 0) return 1;
+  return n * factorial(n - 1);
+};
+
+module.exports = factorial;

13_factorial/solution/factorial-solution.spec.js

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+const factorial = require("./factorial-solution");
+
+describe('factorial', () => {
+  test('4th factorial number is 24', () => {
+    expect(factorial(4)).toBe(24);
+  });
+  test('6th factorial number is 720', () => {
+    expect(factorial(6)).toBe(720);
+  });
+  test('10th factorial number is 3628800', () => {
+    expect(factorial(10)).toBe(3628800);
+  });
+  test('15th factorial number is 1307674368000', () => {
+    expect(factorial(15)).toBe(1307674368000);
+  });
+  test('25th factorial number is 1.5511210043330986e+25', () => {
+    expect(factorial(25)).toBe(1.5511210043330986e+25);
+  });
+  test('0th factorial number is 1', () => {
+    expect(factorial(0)).toBe(1);
+  });
+  test('doesn\'t accept negatives', () => {
+    expect(factorial(-25)).toBe(undefined);
+  });
+  test('doesn\'t accept floats', () => {
+    expect(factorial(5.4)).toBe(undefined);
+  });
+  test('doesn\'t accept a number as a string', () => {
+    expect(factorial('5')).toBe(undefined);
+  });
+  test('doesn\'t accept strings', () => {
+    expect(factorial('foo')).toBe(undefined);
+  });
+  test('doesn\'t accept arrays', () => {
+    expect(factorial([5])).toBe(undefined);
+  });
+});