全屏不工作

function enterFullScreen() {
    var elem = document.documentElement;
    if (elem.requestFullscreen) {
        elem.requestFullscreen();
    } else if (elem.mozRequestFullScreen) { // Firefox
        elem.mozRequestFullScreen();
    } else if (elem.webkitRequestFullscreen) { // Chrome, Safari and Opera
        elem.webkitRequestFullscreen();
    } else if (elem.msRequestFullscreen) { // IE/Edge
        elem.msRequestFullscreen();
    }
}

document.getElementById('fullscreenButton').addEventListener('click', function() {
    if (!document.fullscreenElement && !document.mozFullScreenElement && !document.webkitFullscreenElement && !document.msFullscreenElement) {
        enterFullScreen();
    } else {
        exitFullScreen();
    }
});