ReadableStreamDefaultReader

The ReadableStreamDefaultReader interface is the default reader of the Streams API in Azion Runtime. It reads the data of a stream that comes from the network, such as the body of a response to a fetch() request. A ReadableStreamBYOBReader works only with a stream that has a byte source, but a ReadableStreamDefaultReader reads from any type of ReadableStream source. For more information, refer to ReadableStreamDefaultReader on MDN Web Docs.

Constructor

The ReadableStreamDefaultReader() constructor creates and returns a reader for the stream you pass to it:

new ReadableStreamDefaultReader(stream)

The getReader() method of a ReadableStream also returns a ReadableStreamDefaultReader and locks the stream to it.

Properties

Property	Type	Description
`closed`	Promise	Fulfills when the stream closes. Rejects when the stream throws an error or when the lock of the reader is released. Use it to run code when the stream ends.

Methods

Method	Description
`reader.cancel(reason)`	Returns a promise that resolves when the stream is canceled. A call signals that the consumer has lost interest in the stream. The underlying source receives `reason` and can use it or ignore it.
`reader.read()`	Returns a promise that gives access to the next chunk in the internal queue of the stream, as an object with `value` and `done`. After the last chunk, `done` is `true` and `value` is `undefined`.
`reader.releaseLock()`	Releases the lock of the reader on the stream. After the call, the `locked` property of the stream is `false`.

Example

This handler fetches a page, reads its body one chunk at a time with a reader, and adds each chunk to a stream it constructs. It returns that stream as the body of a Response with an HTML Content-Type header:

JavaScript

01export default {

02 async fetch(request, env, ctx) {

03 return fetch("https://www.example.org/").then((response) => {

04 const reader = response.body.getReader();

05 const stream = new ReadableStream({

06 start(controller) {

07 // Handle each chunk of data

08 function push() {

09 // "done" is a Boolean and "value" is a Uint8Array

10 return reader.read().then(({ done, value }) => {

11 // No more data to read: close the stream

12 if (done) {

13 controller.close();

14 return;

15 }

17 // Pass the chunk to the client through the controller

18 controller.enqueue(value);

19 push();

20 });

21 }

23 push();

24 }

25 });

27 return new Response(stream, { headers: { "Content-Type": "text/html" } });

28 });

29 },

30};

The response that the function returns has these values: its status, its content-type header, the length of its body in characters, and the first 15 characters of the body:

JSON

01{

02 "status": 200,

03 "contentType": "text/html",

04 "bodyChars": 577,

05 "startsWith": "<!doctype html>"

06}

ReadableStream

The stream that a reader locks, and the methods that pipe, split, and cancel it.

ReadableStreamBYOBReader

The reader that reads a byte stream into a buffer you supply.

Response