Firstly, use fetch directly, since your getJustBlabData function is essentially just return fetch(url) wrapped in a new Promise - but since fetch already returns a Promise, there is no need for that code at all

i.e.

function getJustBlabData(url) {
  return new Promise(resolve => {
      const getData = fetch(url);
      resolve(getData)
  })
}
getJustBlabData(url).then ....

is equivalent (but in a lot of ways worse for error handling) to

fetch(url).then ....

If you want to do something every X seconds, do the whole thing in the interval

Like this

setInterval(() => {
  this.loading = true;
  fetch('https://justblab.com/baxf/widget.php?q=online_usr')
  .then((res) => res.json())
  .then((data) => {
      this.justBlab = data;
      this.loading = false;
      console.log(this.justBlab);
  });
}, 1000);

Now, this.justBlab gets updated every second

Note: Also, I've flattened your .then pyramid into a .then chain

to address the situation where requests could take longer than a second

(() => {
    const processBlab = () => {
    this.loading = true;
    fetch('https://justblab.com/baxf/widget.php?q=online_usr')
    .then((res) => res.json())
    .then((data) => {
        this.justBlab = data;
        this.loading = false;
        console.log(this.justBlab);
        setTimeout(() => processBlab(), 1000);
    });
    processBlab();
})();

Only reason I put that all in a IIFE is to preserve this - as there is no context in the question as to what this is

You can use a promise to create a "wait" function and the restart the process again (get a new set of data) once that completes. Here I used a 5 second delay between each cycle.

Doing it this way, if the server takes 10 seconds to reply this will do a new request 5 seconds after that response and avoids piling up requests.

let delayMSTime = 5000;
let howMany = 1;

function sleep(time) {
  return new Promise((resolve) => setTimeout(resolve, time));
}

function getJustBlabData(url) {
  return new Promise(resolve => {
    const getData = fetch(url);
    resolve(getData);
  })
}

function GoGetEm(myDelay) {
  getJustBlabData('https://justblab.com/baxf/widget.php?q=online_usr')
    .then((res) => res.json())
    .then((data) => {
      this.justBlab = data;
      this.loading = false;
      console.log(this.justBlab);
    }).then(() => {
      console.log("sleeeping for the "+howMany+" time:" + myDelay);
      sleep(myDelay).then(() => {
      howMany++;
        GoGetEm(myDelay);
        console.clear();
      });
    });
}
GoGetEm(delayMSTime);