jquery使用ajax实现实时刷新，轮询

来自：https://blog.csdn.net/qq_25406669/article/details/78343082

var isLoaded = false;
function reqs() {
    $.ajax({
        type: 'get',
        url: 'demo.php',
        dataType: 'json',
        beforeSend: function() {
            isLoaded = false;
        },
        success: function(res) {
            console.log(res);
        },
        complete: function() {

            isLoaded = true;
        },
        error: function() {
            console.log('请求失败~');
        }
    });
}
reqs();
setInterval(function() {
    isLoaded && reqs();
}, 3000);