Sending and Receiving Binary Data

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. This can be done with the ImageDecoder interface.

const req = new XMLHttpRequest();
req.open("GET", "/myfile.png", true);
req.responseType = "arraybuffer";

req.onload = (event) => {
  const arrayBuffer = req.response; // Note: not req.responseText
  if (arrayBuffer) {
    const byteArray = new Uint8Array(arrayBuffer);
    byteArray.forEach((element, index) => {
      // do something with each byte in the array
    });
  }
};

req.send(null);

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

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

req.onload = (event) => {
  const blob = req.response;
  // ...
};

req.send();

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.

const req = new XMLHttpRequest();
req.open("POST", url, true);
req.onload = (event) => {
  // Uploaded
};

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

req.send(blob);

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

// Create a new array with fake data (Consecutive numbers (0 - 255), looping back to 0)
const array = new Uint8Array(512).map((v, i) => i);

const xhr = new XMLHttpRequest();
xhr.open("POST", url, false);
xhr.send(array);

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.

See FormData.