Add GCD function and unit tests

.gitignore

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+node_modules/ 

src/utils/gcd.js

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+/**
+ * Author: Aisha Keller
+ * Date: 2 June 2026
+ * File: gcd.js
+ * Description: This script calculates the greatest common divisor of two numbers
+ */
+
+'use strict';
+
+// The gcd function calculates the greates common divisor of two numbers
+function gcd(a, b) {
+
+// If either number is negative, throw an error
+if (a < 0 || b < 0) {
+  throw new Error('GCD of negative numbers is not defined');
+}
+
+// Use the Euclidean alorithm to calculate the GCD
+while (b !== 0) {
+  let temp = b;
+  b = a % b;
+  a = temp;
+}
+
+return a; // Return the GCD
+}
+
+module.exports = { gcd }; // Export the gcd function for use in other scripts

test/utils/gcd.spec.js

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+/**
+ * Author: Aisha Keller
+ * Date: 2 June 2026
+ * File: gcd.spec.js
+ * Description: This script tests the gcd function.
+ */
+
+'use strict';
+
+const { gcd } = require('../../src/utils/gcd'); // Import the gcd function from the gcd.js file
+
+// The describe() function is a test suite that contains one or more tests
+describe('gcd.js', () => {
+
+// Test 1: The it() should calculate the GCD of 12 and 8
+it('should calculate the GCD of 12 and 8', () => {
+  const result = gcd(12, 8); // Call the gcd function with the values of 12 and 8;
+  expect(result).toBe(4); // The expected result is 4
+});
+
+// Test 2: The it() should calculate the GCD of 100 and 25
+it('should calculate the GCD of 100 and 25', () => {
+  const result = gcd(100, 25); // Call the gcd function with the values of 100 and 25
+  expect(result).toBe(25); // The expected result is 25
+});
+
+// Test 3: The it() should throw an error when calculating the GCD of a negative number
+it('should throw an error when calculating the GCD of a negative number', () => {
+  // Call the gcd function with the values of -12 and 8 and expect it to throw an error
+  expect(() => gcd(-1, 5)).toThrow('GCD of negative numbers is not defined');
+});
+});