HTML5-XMLHttpRequest Level 2 overview and detailed explanation

1. Overview
XMLHttpRequest Level 2 is an enhancement to XMLHttpRequest with cross-origin support.

2. Browser support detection

 if (typeof xhr.withCredentials === undefined) 
   {   
      document.getElementById("support").innerHTML =  
         "Your browser <strong>doesnot</strong> support cross-origin                     
         XMLHttpRequest"; 
   } 
   else 
   { 
      document.getElementById("support").innerHTML =  
         "Your browser <strong>does</strong> support cross-origin                        
         XMLHttpRequest";
   }

3. New event name
Before XMLHttpRequest Level 2, XMLHttpRequestrequest and response ##StatusUse some numerical values ​​to represent, similar to enumerations. XMLHttpRequest Level 2 uses named events to replace the different states of request and response. These named events have corresponding event attributes , and event processing functions can be assigned to each event event attributes. XMLHttpRequest
Object has some events, such as: loadstart, progress, abort, error, load, upload, loaded, etc.

These events can be regarded as sub-objects of XMLHttpRequest, and they have some event attributes.

XMLHttpRequest itself has some event attributes and events. XMLHttpRequest event attributes include XMLHttpRequest.onprograss, XMLHttpRequest.onload, XMLHttpRequest.onerror, etc.
You can pass in a parameter e in the event handler function. e has some properties related to events and data information, such as: e.total, e.loaded, e.uploaded, e.downloaded, e.lengthComputable, etc. .

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