Web/API/ReadableStreamDefaultReader/read

The read() method of the ReadableStreamDefaultReader interface returns a promise providing access to the next chunk in the stream's internal queue.

Syntax

var promise = readableStreamDefaultReader.read();

Parameters

None.

Return value

A Promise, which fulfills/rejects with a result depending on the state of the stream. The different possibilities are as follows:

If a chunk is available, the promise will be fulfilled with an object of the form { value: theChunk, done: false }.
If the stream becomes closed, the promise will be fulfilled with an object of the form { value: undefined, done: true }.
If the stream becomes errored, the promise will be rejected with the relevant error.

Exceptions

TypeError: The source object is not a ReadableStreamDefaultReader, or the stream has no owner.

Examples

Example 1 - simple example

This example shows the basic API usage, but doesn't try to deal with complications like stream chunks not ending on line boundaries for example.

In this example stream is a previously-created custom ReadableStream. It is read using a ReadableStreamDefaultReader created using getReader(). (see our [[../../../../../../../mdn.github.io/dom-examples/streams/simple-random-stream/index|Simple random stream example]] for the full code). Each chunk is read sequentially and output to the UI as an array of UTF-8 bytes, until the stream has finished being read, at which point we return out of the recursive function and print the entire stream to another part of the UI.

function fetchStream() {
  const reader = stream.getReader();
  let charsReceived = 0;

  // read() returns a promise that resolves
  // when a value has been received
  reader.read().then(function processText({ done, value }) {
    // Result objects contain two properties:
    // done  - true if the stream has already given you all its data.
    // value - some data. Always undefined when done is true.
    if (done) {
      console.log("Stream complete");
      para.textContent = result;
      return;
    }

    // value for fetch streams is a Uint8Array
    charsReceived += value.length;
    const chunk = value;
    let listItem = document.createElement('li');
    listItem.textContent = 'Received ' + charsReceived + ' characters so far. Current chunk = ' + chunk;
    list2.appendChild(listItem);

    result += chunk;

    // Read some more, and call this function again
    return reader.read().then(processText);
  });
}

Example 2 - handling text line by line

This example shows how you might fetch a text file and handle it as a stream of text lines. It deals with stream chunks not ending on line boundaries and converting from Uint8Array to strings.

async function* makeTextFileLineIterator(fileURL) {
  const utf8Decoder = new TextDecoder("utf-8");
  let response = await fetch(fileURL);
  let reader = response.body.getReader();
  let {value: chunk, done: readerDone} = await reader.read();
  chunk = chunk ? utf8Decoder.decode(chunk) : "";

  let re = /\r\n|\n|\r/gm;
  let startIndex = 0;
  let result;

  for (;;) {
    let result = re.exec(chunk);
    if (!result) {
      if (readerDone) {
        break;
      }
      let remainder = chunk.substr(startIndex);
      ({value: chunk, done: readerDone} = await reader.read());
      chunk = remainder + (chunk ? utf8Decoder.decode(chunk) : "");
      startIndex = re.lastIndex = 0;
      continue;
    }
    yield chunk.substring(startIndex, result.index);
    startIndex = re.lastIndex;
  }
  if (startIndex < chunk.length) {
    // last line didn't end in a newline char
    yield chunk.substr(startIndex);
  }
}

for await (let line of makeTextFileLineIterator(urlOfFile)) {
  processLine(line);
}

Specifications

Specification	Status	Comment
StreamsThe definition of 'read()' in that specification.	Living Standard	Initial definition.

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
`read` Experimental'	Chrome ?	Edge ?	Firefox Full support 65 Full support 65 Full support 57 Disabled' Disabled' From version 57: this feature is behind the `dom.streams.enabled` preference (needs to be set to `true`) and the `javascript.options.streams` preference (needs to be set to `true`). To change preferences in Firefox, visit about:config.	IE No support No	Opera ?	Safari ?	WebView Android ?	Chrome Android ?	Firefox Android Full support 65 Full support 65 Full support 57 Disabled' Disabled' From version 57: this feature is behind the `dom.streams.enabled` preference (needs to be set to `true`) and the `javascript.options.streams` preference (needs to be set to `true`). To change preferences in Firefox, visit about:config.	Opera Android ?	Safari iOS ?	Samsung Internet Android ?

Legend

Full support: Full support
No support: No support
Compatibility unknown: Compatibility unknown
Experimental. Expect behavior to change in the future.': Experimental. Expect behavior to change in the future.
User must explicitly enable this feature.': User must explicitly enable this feature.

ReadableStreamDefaultReader.read() by Mozilla Contributors is licensed under CC-BY-SA 2.5.