網頁全屏按鈕代碼

let fullscreen=false //處於未全屏狀態
 let element = document.documentElement;
function openFull(){       
     if (fullscreen) {
                if (document.exitFullscreen) {
                    document.exitFullscreen();
                } else if (document.webkitCancelFullScreen) {
                    document.webkitCancelFullScreen();
                } else if (document.mozCancelFullScreen) {
                    document.mozCancelFullScreen();
                } else if (document.msExitFullscreen) {
                    document.msExitFullscreen();
                }
            } else {
                if (element.requestFullscreen) {
                    element.requestFullscreen();
                } else if (element.webkitRequestFullScreen) {
                    element.webkitRequestFullScreen();
                } else if (element.mozRequestFullScreen) {
                    element.mozRequestFullScreen();
                } else if (element.msRequestFullscreen) {
                    element.msRequestFullscreen();
                }
            }
           fullscreen = !fullscreen;}