concept of stub in node.js

const request = require('request');
const getPhotosByAlbumId = (id) => {
const requestUrl = `https://jsonplaceholder.typicode.com/albums/${id}/photos?_limit=3`;
return new Promise((resolve, reject) => {
    request.get(requestUrl, (err, res, body) => {
        if (err) {
            return reject(err);
        }
        resolve(JSON.parse(body));
    });
});
};
module.exports = getPhotosByAlbumId;
To test this function this is the stub
const expect = require('chai').expect;
const request = require('request');
const sinon = require('sinon');
const getPhotosByAlbumId = require('./index');
describe('with Stub: getPhotosByAlbumId', () => {
before(() => {
    sinon.stub(request, 'get')
        .yields(null, null, JSON.stringify([
            {
                "albumId": 1,
                "id": 1,
                "title": "A real photo 1",
                "url": "https://via.placeholder.com/600/92c952",
                "thumbnailUrl": "https://via.placeholder.com/150/92c952"
            },
            {
                "albumId": 1,
                "id": 2,
                "title": "A real photo 2",
                "url": "https://via.placeholder.com/600/771796",
                "thumbnailUrl": "https://via.placeholder.com/150/771796"
            },
            {
                "albumId": 1,
                "id": 3,
                "title": "A real photo 3",
                "url": "https://via.placeholder.com/600/24f355",
                "thumbnailUrl": "https://via.placeholder.com/150/24f355"
            }
        ]));
});
after(() => {
    request.get.restore();
});
it('should getPhotosByAlbumId', (done) => {
    getPhotosByAlbumId(1).then((photos) => {
        expect(photos.length).to.equal(3);
        photos.forEach(photo => {
            expect(photo).to.have.property('id');
            expect(photo).to.have.property('title');
            expect(photo).to.have.property('url');
        });
        done();
    });
});
});