<div id="particles-js"></div>
<div id="instances">pJS Instances: <span></span><button type="button">Refresh!</button></div>

#particles-js {
  position: fixed;
  width: 100%;
  height: 100%;
  margin: 0;
  padding: 0;
  top: 0;
  left: 0;
  background: #000;
  z-index: -1;
}

#instances {
  position: fixed;
  left: 30px;
  top: 30px;
  background: #fff;
  color: #000;
  padding: 5px;
  font-family: Verdana;
}

const updateInstances = () => {
  document.querySelector("#instances span").innerText = `${pJSDom.length}`;
};

const refreshParticles = () => {
  particlesJS("particles-js", {
    interactivity: {
      detect_on: "window"
    },
    particles: {
      number: {
        value: 200
      },
      move: {
        speed: 6
      },
      size: {
        value: 1
      }
    }
  });

  updateInstances();
};

refreshParticles();

document
  .querySelector("#instances button")
  .addEventListener("click", refreshParticles);

console.log(pJSDom[0]);