How can I make this function asynchronous? Or an even better solution

So, I am making a fake operating system game inside the console in JS. And to load the OS I have a twirly loading function which I'll show now.

function OSLoad(OS = "", loadingTimeSeconds = 10) {
  var twirlTimer = (function () {
    var P = ["\\", "|", "/", "-"];
    var x = 0;
    return setInterval(function () {
      process.stdout.write(
        "\r" + P[x++] + " Loading " + OS
      );
      x &= 3;
    }, 250);
  })();

  setTimeout(() => clearInterval(twirlTimer), loadingTimeSeconds + "000");
}

I'm kind of a beginner, and I've ran into a problem with this function. I'm currently in the testing phase, so after this function is done I log to the console "OS Started!", but instead it logs the loading and "OS Started!" at the same time. I assumed that setTimeout would make it pause until it's done, which it didn't. What can I do to solve this?

The first thing that came to mind when I tried to solve this was using async, but as I said, I'm a beginner, and I never actually learned how to use async... So if you could teach me how to use that or an even better solution please do.

The best way in JS to make code in JS look sync is using async / await.

Instead of using multiple setTimeout , intervals etc, your can create a simple Promise sleep function and then use this with await.

Below is a simple example. Most the code is to emulate the console.

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

//very simple console emulator
const pre = document.querySelector('pre');
const consoleBuffer = new Array(10).fill(' ');
let cpos = 0;
const consoleWrite = txt => {
  const s = txt.split('');
  for (const c of s) {
    if (c === '\r') cpos = 0;
    else consoleBuffer[cpos++] = c;
  }
  pre.innerText = consoleBuffer.join('');
}
window.process = { stdout: { write: consoleWrite } };


//OS Loader
const sleep = ms => new Promise(r => { setTimeout(r, ms);});

async function OSLoad(OS = "", loadingTimeSeconds = 10) {
  var P = ["\\", "|", "/", "-"];
  var x = 0;
  for (let lp = 0; lp < loadingTimeSeconds * 4; lp ++) {
    process.stdout.write(
      "\r" + P[x++] + " Loading " + OS
    );
    x &= 3;
    await sleep(250);
  }
  process.stdout.write("\r" + OS + " is loaded..");
}


OSLoad('Fake OS 4.01');

<!-- language: lang-css -->

pre {
  height: 90vh;
  background-color: black;
  color: white;
}

<!-- language: lang-html -->

<pre>
</pre>

<!-- end snippet -->