filereader - javascript - how to retireve bytes of file read using readAsArrayBuffer -

i reading file using filereader's readasarraybuffer. once file read, want cache content can submit server when user saves whatever doing.

here's code

getfilebuffer: function(file) {   var deferred = $.deferred(),reader = new filereader();   reader.onload = function(e) {    deferred.resolve(e.target.result);   };   reader.onerror = function(e) {    deferred.reject(e.target.error);   };   reader.readasarraybuffer(file);     return deferred;   }; } 

and then

_.each(evt.target.files, function(file) {    $this.getfilebuffer(file).done(function(cont) {      attachment[f.filename]=cont;     }); }); 

how retrieve binary content later? when access cached object, arraybuffer object single bytelength property. reading dataview don't know how actual bytes dataview store them.