How does jquery determine whether the iframe is loaded?

var frm = document.getElementById('myiframe');  
$(frm).load(function(){                             //  等iframe加载完毕  
dosomething();  
});

var iframe = document.createElement("myiframe");  
iframe.src = "http://www.baidu.com";  
if (!/*@cc_on!@*/0) { //如果不是IE，IE的条件注释  
    iframe.onload = function(){     
        alert("Local iframe is now loaded.");  
    };  
} else {  
    iframe.onreadystatechange = function(){ // IE下的节点都有onreadystatechange这个事件  
        if (iframe.readyState == "complete"){  
            alert("Local iframe is now loaded.");  
        }  
    };  
}  
document.body.appendChild(iframe);

var iframe = document.createElement("iframe");  
iframe.src = "http://www.baidu.com";  
  
if (iframe.attachEvent){  
    iframe.attachEvent("onload", function(){ // IE  
        alert("Local iframe is now loaded.");  
    });  
} else {  
    iframe.onload = function(){ // 非IE  
        alert("Local iframe is now loaded.");  
    };  
}  
  
document.body.appendChild(iframe);