An example of unit test for a simple main file

index.spec.ts

import { stub, SinonStub } from 'sinon';
import * as server from '../src/server';

// First describe, class name
describe('index.ts', () => {
  let start: SinonStub;

  // Before each to bootstrap stubs, where all the methods that could be called are stubbeds
  beforeEach(() => {
    start = sinon.stub(server, 'start');
    // here we clean the require cache, to make sure index will not be loaded before our test
    delete require.cache[require.resolve('../src/index')];
  });

  // A single test case
  it('should do something when something happens', () => {
    const result = require('../src/index');
    expect(start).to.have.been.calledOnceWithExatly();
    expect(result).to.be.undefined;
  });
});