XHR2：js异步上传

http://dev.opera.com/articles/xhr2/

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var xhr = new XMLHttpRequest();   var onProgressHandler = function(event) {   if(event.lengthComputable) {     var howmuch = (event.loaded / event.total) * 100;     document.querySelector('progress').value = Math.ceil(howmuch);   } else {     console.log("Can't determine the size of the file.");   } }   var onLoadHandler = function() {   displayLoadedMessage(); }   var onErrorHandler = function() {   displayErrorMesssage(); }   xhr.upload.addEventListener('progress', onProgressHandler, false); xhr.upload.addEventListener('load', onLoadHandler, false); xhr.upload.addEventListener('error', onErrorHandler, false);   var onReadyStateHandler = function(event) {   if( event.target.readyState == 4 && event.target.status == 200){        } }   xhr.open('POST','/path_to_data'); xhr.onreadystatechange = onReadyStateHandler; xhr.send(dataToSend);