TDD - Greatest Common Divisor (https://learn.freecodecamp.org/coding-interview-prep/rosetta-code/greatest-common-divisor)

gcd.js

module.exports = function gcd(a, b) {
    let arra = [];
    let arrb = []

    for (let i=0; i<= a; i++){
        if (a%i ===0) {
            arra.push(i);
        }
    }
    for (let j=0; j<= b; j++){
        if (b%j ===0) {
            arrb.push(j);
        }
    }
    // arra = [1,2,5], arrb = [1,3,5]

    let common = arra.filter(item => arrb.includes(item)); // x3 , common = []
    // 1-> item = 1 ---> arrb.includes(1) ?? ===> true ---> common [1]
    // 2-> item = 2 ---> arrb.includes(2) ?? ===> false --> common [1]
    // 3-> item = 5 ---> arrb.includes(5) ?? ===> true ---> common [1,5]
    return common.pop(); // quita el ultimo elemento, primero usa .shift()
} 

// otra solucion mas corta
module.exports = function gcd(a, b) {
  let result;            // PASS 1                   // PASS 2                       // PASS 3
  while ((a % b) > 0) {  // a = 12, b = 18, % = 12   // a = 18, b = 12, % = 6        // a = 12, b = 6, % = 0
    result = a % b;      // result = 12              // result = 6                   // -> sale , result = 6
    a = b;               // a = 18                   // a = 12
    b = result;          // b = 12                   // b = 6
  }
  return b;
};

gcd.test.js

const assert = require ('assert');
const gcd = require ('./gcd.js');

describe ('Testing GCD', () => {

// gcd should be a function

    it('should be a function', () => {
        assert.equal(typeof gcd, 'function');
    })

// gcd(24,36) should return a number

     it('should return "number" when encoding 24, 36', () => {
         assert.equal(typeof gcd(24,36), 'number');
     })

// gcd(24,36) should return 12

    it('should return "number" when encoding 24, 36', () => {
     assert.equal(gcd(24,36), 12);
     })

// gcd(30,48) should return 6

    it('should return "6" when encoding (30,48) ', () => {
    assert.equal(gcd(30,48), 6);
    })

 // gcd(10,15) should return 5

    it('should return "5" when encoding (10,15)', () => {
    assert.equal(gcd(10,15), 5);
    })

// gcd(100,25) should return 25

    it('should return "25" when encoding (100,25)', () => {
    assert.equal(gcd(100,25), 25);
    })

// gcd(13,250) should return 1

    it('should return "1" when encoding (13,250)', () => {
    assert.equal(gcd(13, 250), 1);
    });

//gcd(1300,250) should return 50

    it('should return "50" when encoding (1300,250)', () => {
    assert.equal(gcd(1300, 250), 50);
    });

});

To get started:

$ mkdir gcd_dojo

$ cd gcd_dojo

$ npm init -y
Wrote to /projects/wcs/gcd_dojo/package.json:

{
  "name": "gcd_dojo",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC"
}

$ npm i mocha

npm notice created a lockfile as package-lock.json. You should commit this file.
npm WARN gcd_dojo@1.0.0 No description
npm WARN gcd_dojo@1.0.0 No repository field.

+ mocha@6.1.4
added 115 packages from 509 contributors and audited 224 packages in 17.868s
found 0 vulnerabilities

$ code .

Then continue in VSCode and execute npm test from inbuilt terminal.