feat(factorial): create exercise

13_factorial/README.md

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+# Exercise 13 - Factorial
+
+Create a function that takes an input integer greater than or equal to 1 and returns the product of all its predecessing integers. If 0 is input it returns 1. It should accept strings that when converted to a number are valid inputs. For invalid inputs it returns `undefined`.
+
+For example:
+
+```javascript
+factorial(2); // 2 * 1, Output: 2
+factorial('4'); // 4 * 3 * 2 * 1, Output: 24
+factorial(5); // 5 * 4 * 3 * 2 * 1, Output: 120
+factorial(7.2); // Output: undefined
+factorial(0); // Output: 1
+```

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-solution");
+
+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('DOES accept strings', () => {
+    expect(factorial("0")).toBe(1);
+  });
+  test.skip('DOES accept strings', () => {
+    expect(factorial("1")).toBe(1);
+  });
+  test.skip('DOES accept strings', () => {
+    expect(factorial("2")).toBe(2);
+  });
+  test.skip('DOES accept strings', () => {
+    expect(factorial("8")).toBe(40320);
+  });
+});

13_factorial/solution/factorial-solution.js

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+const factorial = function(n) {
+  // a negative number will always contain a '-'
+  // a non-integer will always contain a '.'
+  // the [-.] regex literal will match strings that contain a '-' or a '.'
+  // n + '' converts n to a string, which is required for the Regex.prototype.test method 
+  if (/[-.]/.test(n + '')) return;
+
+  // +n converts n to a number
+  if (+n === 0) return 1;
+  return n * factorial(n - 1);
+};
+
+// Do not edit below this line
+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.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('DOES accept strings', () => {
+    expect(factorial("0")).toBe(1);
+  });
+  test.skip('DOES accept strings', () => {
+    expect(factorial("1")).toBe(1);
+  });
+  test.skip('DOES accept strings', () => {
+    expect(factorial("2")).toBe(2);
+  });
+  test.skip('DOES accept strings', () => {
+    expect(factorial("8")).toBe(40320);
+  });
+});