Only IE9/10 supports both onload and onreadystatechange event analysis of script elements

as follows

Copy the codeThe code is as follows:

 
<!DOCTYPE HTML> 
<html> 
<head> 
<meta charset="utf-8"> 
<title>IE9/10 supports both onload and onreadystatechange events of script elements</title>
<script src="/" onload="alert(1)" onreadystatechange="alert(2)"></script> 
</head> 
<body> 
</body> 
</html> 

result:
IE6/7/8: Popup 2
IE9/10: Popup 2, 1
Firefox/Safari/Chrome/Opera: Popup 1
As shown in the test results, the script's onload event has begun to be supported after IE9. We have always used the above two events to determine whether the js file has been loaded. I knew a long time ago that onreadystatechange event is used in IE, and the value of readyState is used in the event handler to determine whether the loading is completed. Other browsers use the onload event.

Copy the codeThe code is as follows:

 
if(isIE){ 
 = function(){ 
if( == 'loaded' ||  == 'complete'){ 
callback(); 
} 
} 
}else{ 
 = function(){ 
callback(); 
} 
} 

There is no problem with this writing now. However, the following writing method may cause the callback to be executed twice in IE9/10

Copy the codeThe code is as follows:

 
 =  = function(){ 
if(! ||  == "loaded" ||  == "complete"){ 
callback(); 
} 
} 

The trick of this writing method is that both onload and onreadystatechage use the same function. Firefox/Safari/Chrome/Opera does not support the onreadystatechage event, nor does it have the readyState attribute, so ! is for these browsers. readyState is for IE browser. The loaded situation is loaded. When cached, readyState may appear complete. So two cannot be missed. However, since IE9/10 also supports the onload event, the callback will be executed twice.

http:///TR/html401/interact/#h-18.2.1

http:///TR/html5/#script

/En/HTML/Element/Script