Blocking and non blocking code in parallel

I need to be able to run some code that is going to be blocking and some other code that will then, when blocked, start some other actions.

The usecase is the follows:

I have a file called index.ts running an express and socket server

I have a testfile called test.spec.ts that needs to be able to start the express server and then initiate some commands for running tests either via HTTP request or socket message(I would prefer HTTP)

The only way I found to keep the webserver alive is instanciating it with

import { spawnSync } from 'child_process';

spawnSync('ts-node', ['path/to/index.ts"], { cwd: "path/to/workdir"});

which will block until the child process is killed( could be up to 30min later).

Is there a way to split into two processes, one that gets blocked when starting it and one continuing to work that exposes some functions for interactions with the test file?

My target would look like this:

// index.ts

import * as express from "express";
const app = express();

const port = 3000;

app.get('/', (req, res) => {
    res.send('Hello World!');
});

app.listen(port, () => {
    console.log(`Example app listening on port ${port}`);
});

// test.spec.ts

import { spawnSync } from 'child_process';

describe("Test",()=>{

  it("Test", async ()=>{

    // create somehow a child process that should block
    const childProcess = ...
    childProcess.do(spawnSync('ts-node', ['path/to/index.ts'], {cwd: 'path/to/workdir'}) //should block now

    // the following code should run in parallel
    await new Promise(r => setTimeout(r, 5000)); //wait some time until the webserver is ready

    fetch('http://localhost:3000').then((ret)=>{
      expect(ret,'to be Hello World').to.contain('Hello World!");
    })
    ... // more tests
  });
});