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Response: body property

The body read-only property of the Response interface is a ReadableStream of the body contents.

Value

A ReadableStream, or else null for any Response object constructed with a null body property, or for any actual HTTP response that has no body.

The stream is a readable byte stream, which supports zero-copy reading using a ReadableStreamBYOBReader.

Note: Current browsers don't actually conform to the spec requirement to set the body property to null for responses with no body (for example, responses to HEAD requests, or 204 No Content responses).

Examples

Copying an image

In our simple stream pump example we fetch an image, expose the response's stream using response.body, create a reader using ReadableStream.getReader(), then enqueue that stream's chunks into a second, custom readable stream — effectively creating an identical copy of the image.

const image = document.getElementById("target");

// Fetch the original image
fetch("./tortoise.png")
  // Retrieve its body as ReadableStream
  .then((response) => response.body)
  .then((body) => {
    const reader = body.getReader();

    return new ReadableStream({
      start(controller) {
        return pump();

        function pump() {
          return reader.read().then(({ done, value }) => {
            // When no more data needs to be consumed, close the stream
            if (done) {
              controller.close();
              return;
            }

            // Enqueue the next data chunk into our target stream
            controller.enqueue(value);
            return pump();
          });
        }
      },
    });
  })
  .then((stream) => new Response(stream))
  .then((response) => response.blob())
  .then((blob) => URL.createObjectURL(blob))
  .then((url) => console.log((image.src = url)))
  .catch((err) => console.error(err));

Creating a BYOB reader

In this example we construct a ReadableStreamBYOBReader from the body using ReadableStream.getReader({mode: 'byob'}). We can then use this reader to implement zero copy transfer of the response data.

async function getProducts(url) {
  const response = await fetch(url);
  const reader = response.body.getReader({ mode: "byob" });
  // read the response
}

getProducts(
  "https://mdn.github.io/learning-area/javascript/apis/fetching-data/can-store/products.json",
);

Specifications

Specification
Fetch Standard # ref-for-dom-body-body①

Browser compatibility