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();
oReq.open("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
    }
  }
};

oReq.send(null);

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

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

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

oReq.send();

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();
  req.open('GET', url, false);
  //XHR binary charset opt by Marcus Granado 2006 [http://mgran.blogspot.com]
  req.overrideMimeType('text\/plain; charset=x-user-defined');
  req.send(null);
  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.

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 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();
oReq.open("POST", url, true);
oReq.onload = function (oEvent) {
  // Uploaded.
};

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

oReq.send(blob);

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);

// generate some data
for (var i=0; i< longInt8View.length; i++) {
  longInt8View[i] = i % 256;
}

var xhr = new XMLHttpRequest;
xhr.open("POST", url, false);
xhr.send(myArray);

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.

Submitting forms and uploading files

Please, read this paragraph.