How to get blob data using javascript XmlHttpRequest by sync & how to change the blob type


Firefox 33+ OK

Chrome 38+ OK

IE 6 — IE 10 Failed


Thanks to 阮一峰’s blog:



The sample shows how to get blob data using javascript XmlHttpRequest by sync.


The w3c tell us cannot set responseType when async is false.

If async is false, the JavaScript global environment is a document environment, and either the anonymous flag is set, the timeout attribute value is not zero, the withCredentials attribute value is true, or the responseType attribute value is not the empty string, throw an “InvalidAccessError” exception and terminate these steps.


The result is that the type of XMLHttpRequest.response always string, you must change string to blob.
if you not set “charset=x-user-defined”, the string is ascii by default. The XMLHttpRequest.response is not correct, some bytes are changed.
I set to utf-8 or utf-16 also for test, but failed.
when using utf-8, the length of XMLHttpRequest.response is not correct
when using utf-16, the length equals to Real-Length/2. the problem is when Real-Length is odd I cannot get the last byte.



<div id=”aabb”></div>

var URL = URL || window;

function saveAs(blob, filename) {
var type = blob.type;
var force_saveable_type = ‘application/octet-stream’;

// tips: how to change the blob type? follow me –>
if (type && type != force_saveable_type) {
    var slice = blob.slice || blob.webkitSlice || blob.mozSlice;
    blob =, 0, blob.size, force_saveable_type);

var url = URL.createObjectURL(blob);
var save_link = document.createElementNS(‘’, ‘a’);
save_link.href = url; = filename;
save_link.text = ‘ffff’;

// in firefox 33+
var m = document.getElementById(‘aabb’);
m.innerHTML = ‘<a href=”‘ +url+ ‘”>FFF</a>’;

// in chrome 38+
var event = document.createEvent(‘MouseEvents’);
event.initMouseEvent(‘click’, true, false, window, 0, 0, 0, 0, 0, false, false, false, false, 0, null);

function binaryToBlob(data) {
var l = data.length, arr = new Uint8Array(l);

for(var i = 0; i < l; i++) {
arr[i] = data.charCodeAt(i);

return new Blob([arr], { type: ‘image/octet-stream’})

var r = new XMLHttpRequest();“GET”, ‘a.tif’, false);
r.overrideMimeType(‘text/plain; charset=x-user-defined’); // this line is very important ,the charset must be x-user-defined

var blob = binaryToBlob(r.response);
blob.fileType = “image/octet-stream”;
saveAs(blob, ‘res.tif’);