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);