The Tor Browser / file revision

 /**

  * Expects an object containing a blob, a start offset, an end offset

  * and an optional content type to slice the blob. Returns an object

  * containing the size and type of the sliced blob.

  */

 onmessage = function(event) {

   var blob = event.data.blob;

   var start = event.data.start;

   var end = event.data.end;

   var contentType = event.data.contentType;

   var slicedBlob;

   if (contentType == undefined && end == undefined) {

     slicedBlob = blob.slice(start);

   } else if (contentType == undefined) {

     slicedBlob = blob.slice(start, end);

   } else {

     slicedBlob = blob.slice(start, end, contentType);

   }

   var rtnObj = new Object();

   rtnObj.size = slicedBlob.size;

   rtnObj.type = slicedBlob.type;

   postMessage(rtnObj);

 };