feat: add first factorial solution

Author: marcode24

coderbyte/first-factorial/README.md

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+# First Factorial
+
+## Description
+
+Have the function FirstFactorial(num) take the num parameter being passed and return the factorial of it. For example: if num = 4, then your program should return (4 _ 3 _ 2 \* 1) = 24. For the test cases, the range will be between 1 and 18 and the input will always be an integer.

coderbyte/first-factorial/solution.js

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+function firstFactorial(num) {
+  // code goes here
+  let result = 1;
+  while (num > 0) {
+    result *= num;
+    num--;
+  }
+  return result;
+}
+
+module.exports = firstFactorial;

coderbyte/first-factorial/solution.test.js

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+const firstFactorial = require('./solution');
+
+describe('First Factorial', () => {
+  const testCases = [
+    {
+      input: 4,
+      output: 24,
+    },
+    {
+      input: 8,
+      output: 40320,
+    },
+    {
+      input: 12,
+      output: 479001600,
+    },
+  ];
+
+  it('should return a number type', () => {
+    expect(typeof firstFactorial(4)).toBe('number');
+  });
+
+  it.each(testCases)('should return $output', (testCase) => {
+    expect(firstFactorial(testCase.input)).toBe(testCase.output);
+  });
+});