fullscreen.js

let myDocument = document.getElementById("target");
  let btn = document.getElementById("btn");
  btn.addEventListener("click", () => {
    if (btn.textContent == "Go Fullscreen") {
      if (myDocument.requestFullscreen) {
        myDocument.requestFullscreen();
      } else if (myDocument.msRequestFullscreen) {
        myDocument.msRequestFullscreen();
      } else if (myDocument.mozRequestFullScreen) {
        myDocument.mozRequestFullScreen();
      } else if (myDocument.webkitRequestFullscreen) {
        myDocument.webkitRequestFullscreen();
      }
      btn.textContent = "Exit Fullscreen";
    } else {
      if (document.exitFullscreen) {
        document.exitFullscreen();
      } else if (document.msexitFullscreen) {
        document.msexitFullscreen();
      } else if (document.mozexitFullscreen) {
        document.mozexitFullscreen();
      } else if (document.webkitexitFullscreen) {
        document.webkitexitFullscreen();
      }
      btn.textContent = "Go Fullscreen";
    }
  });