Sending and Receiving Binary Data

Receiving binary data using JavaScript typed arrays

The responseType property of the XMLHttpRequest object can be set to change the expected response type from the server. Possible values are the empty string (default), "arraybuffer", "blob", "document", "json", and "text". The response property will contain the entity body according to responseType, as an ArrayBuffer, Blob, Document, JSON, or string. This is null if the request is not complete or was not successful.

This example reads an image as a binary file and creates an 8-bit unsigned integer array from the raw bytes. Note that this will not decode the image and read the pixels. You will need a png decoding library for that.

var oReq = new XMLHttpRequest();"GET", "/myfile.png", true);
oReq.responseType = "arraybuffer";

oReq.onload = function (oEvent) {
  var arrayBuffer = oReq.response; // Note: not oReq.responseText
  if (arrayBuffer) {
    var byteArray = new Uint8Array(arrayBuffer);
    for (var i = 0; i < byteArray.byteLength; i++) {
      // do something with each byte in the array


An alternative to the above method utilizes the Blob interface to directly construct a Blob with the arraybuffer data.

var oReq = new XMLHttpRequest();"GET", "/myfile.png", true);
oReq.responseType = "arraybuffer";

oReq.onload = function(oEvent) {
  var blob = new Blob([oReq.response], {type: "image/png"});
  // ...


Also you can read a binary file as a Blob by setting the string "blob" to the responseType property.

var oReq = new XMLHttpRequest();"GET", "/myfile.png", true);
oReq.responseType = "blob";

oReq.onload = function(oEvent) {
  var blob = oReq.response;
  // ...


Receiving binary data in older browsers

The load_binary_resource() function shown below loads binary data from the specified URL, returning it to the caller.

function load_binary_resource(url) {
  var req = new XMLHttpRequest();'GET', url, false);
  //XHR binary charset opt by Marcus Granado 2006 []
  req.overrideMimeType('text\/plain; charset=x-user-defined');
  if (req.status != 200) return '';
  return req.responseText;

The magic happens in line 5, which overrides the MIME type, forcing the browser to treat it as plain text, using a user-defined character set. This tells the browser not to parse it, and to let the bytes pass through unprocessed.

var filestream = load_binary_resource(url);
var abyte = filestream.charCodeAt(x) & 0xff; // throw away high-order byte (f7)

The example above fetches the byte at offset x within the loaded binary data. The valid range for x is from 0 to filestream.length-1.

See downloading binary streams with XMLHttpRequest for a detailed explanation. See also downloading files.

Receiving binary data anywhere

jBinary library for working with binary data in JavaScript allows to load data from any source with automatically detected best supported way on current browser or Node.js:

jBinary.load(url).then(function (binary) {
  // here you can use `binary` instance to parse data  
  // in any format (string, array of bytes, custom structure etc.)

Sending binary data

The send method of the XMLHttpRequest has been extended to enable easy transmission of binary data by accepting an ArrayBuffer, Blob, or File object.

The following example sends creates a text file on-the-fly and uses the POST method to send the "file" to the server. This example uses plain text, but you can imagine the data being a binary file instead.

var oReq = new XMLHttpRequest();"POST", url, true);
oReq.onload = function (oEvent) {
  // Uploaded.

var blob = new Blob(['abc123'], {type: 'text/plain'});


Sending typed arrays as binary data

You can send JavaScript typed arrays as binary data as well.

var myArray = new ArrayBuffer(512);
var longInt8View = new Uint8Array(myArray);

for (var i=0; i< longInt8View.length; i++) {
  longInt8View[i] = i % 255;

var xhr = new XMLHttpRequest;"POST", url, false);

This is building a 512-byte array of 8-bit integers and sending it; you can use any binary data you'd like, of course.

Note: Support for sending ArrayBuffer objects using XMLHttpRequest was added to Gecko 9.0 (Firefox 9.0 / Thunderbird 9.0 / SeaMonkey 2.6). Add information about other browsers' support here.

Submitting forms and uploading files

Please, read this paragraph.

Firefox-specific examples

This example transmits binary content asynchronously, using the POST method, and Firefox's non-standard sendAsBinary().

var req = new XMLHttpRequest();"POST", url, true);
// set headers and mime-type appropriately
req.setRequestHeader("Content-Length", 741);

Line 4 sets the Content-Length header to 741, indicating that the data is 741 bytes long.  Obviously you need to change this value based on the actual size of the data being sent.

Line 5 uses the sendAsBinary() method to initiate the request.

Note: This non-standard sendAsBinary method is considered deprecated as of Gecko 31 (Firefox 31 / Thunderbird 31 / SeaMonkey 2.28) and will be removed soon. The standard send(Blob data) method can be used instead as explained above.

You can also send binary content by passing an instance of the nsIFileInputStream to send(). In that case, you don't have to set the Content-Length header yourself, as the information is fetched from the stream automatically:

// Make a stream from a file.
var stream = Components.classes[";1"]
stream.init(file, 0x04 | 0x08, 0644, 0x04); // file is an nsIFile instance   

// Try to determine the MIME type of the file
var mimeType = "text\/plain";
try {
  var mimeService = Components.classes[";1"]
  mimeType = mimeService.getTypeFromFile(file); // file is an nsIFile instance
catch (oEvent) { /* eat it; just use text/plain */ }

// Send    
var req = Components.classes[";1"]
                    .createInstance(Components.interfaces.nsIXMLHttpRequest);'PUT', url, false); /* synchronous! */
req.setRequestHeader('Content-Type', mimeType);


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Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.

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