在阅读Long Polling上的一篇文章时,我对以下两种setInterval之间的问题感到困惑

1 –

setInterval(function(){
    $.ajax({ url: "server",success: function(data){
        //Update your dashboard gauge
        salesGauge.setValue(data.value);
    },dataType: "json"});
},30000);

2-

(function poll() {
   setTimeout(function() {
       $.ajax({ url: "server",success: function(data) {
            sales.setValue(data.value);
       },dataType: "json",complete: poll });
    },30000);
})();

根据博客,它说 – 关于第二个片段,

So,this pattern doesn’t guarantee execution on a fixed interval per
se. But,it does guarantee that the prevIoUs interval has completed
before the next interval is called.

为什么第二个片段保证前一个间隔已经完成？

我知道第一个(事件循环)但很少混淆第二个片段.