mui两次按返回键退出app

<script >
		document.addEventListener('plusready', function() {
		        var webview = plus.webview.currentWebview();
		        plus.key.addEventListener('backbutton', function() {
		            webview.canBack(function(e) {
		                if(e.canBack) {
		                    webview.back();             
		                } else {
		                    //webview.close(); //hide,quit
		                    //plus.runtime.quit();
		                    mui.plusReady(function() {
		                        //首页返回键处理
		                        //处理逻辑：1秒内，连续两次按返回键，则退出应用；
		                        var first = null;
		                        plus.key.addEventListener('backbutton', function() {
		                            //首次按键，提示‘再按一次退出应用’
		                            if (!first) {
		                                first = new Date().getTime();
		                                mui.toast('再按一次退出应用');
		                                setTimeout(function() {
		                                    first = null;
		                                }, 1000);
		                            } else {
		                                if (new Date().getTime() - first < 1500) {
		                                    plus.runtime.quit();
		                                }
		                            }
		                        }, false);
		                    });
		                }
		            })
		        });
		    });
</script>