function FullScreen() {
    let domElement = document.documentElement;
    if (domElement.requestFullscreen) {
        domElement.requestFullscreen();
    } else if (domElement.mozRequestFullScreen) {
        domElement.mozRequestFullScreen();
    } else if (domElement.webkitRequestFullScreen) {
        domElement.webkitRequestFullScreen();
    }
};

function CancelFullscreen() {
    let domElement = document;
    if (domElement.exitFullscreen) {
        domElement.exitFullscreen();
    } else if (domElement.mozCancelFullScreen) {
        domElement.mozCancelFullScreen();
    } else if (domElement.webkitCancelFullScreen) {
        domElement.webkitCancelFullScreen();
    }
}

var flag = true;
const clickFunction = () => {
  if(flag){
  FullScreen();
  flag = false;
  document.querySelector('#btn').innerHTML="退出全屏";
  }else{
  CancelFullscreen();
  flag = true;
  document.querySelector('#btn').innerHTML="进入全屏";
  }
 }
 <button onclick="clickFunction" id="btn">进入全屏</button>