Web/API/ReadableStream/getReader

The getReader() method of the ReadableStream interface creates a reader and locks the stream to it. While the stream is locked, no other reader can be acquired until this one is released.

Syntax

var reader = readableStream.getReader({mode});

Parameters

{mode} Optional: An object containing a property mode, specifying the type of reader to create. Values can be:

"byob", which results in a ReadableStreamBYOBReader being created that can read readable byte streams (i.e. can handle "bring your own buffer" reading). undefined (or not specified at all — this is the default), which results in a ReadableStreamDefaultReader being created that can read individual chunks from a stream.

Return value

A ReadableStreamDefaultReader or ReadableStreamBYOBReader object instance, depending on the mode value.

Exceptions

RangeError: The provided mode value is not "byob" or undefined.
TypeError: The stream you are trying to create a reader for is not a ReadableStream.

Examples

In the following simple example, a previously-created custom ReadableStream 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, 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 = value;
      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);
  });
}

Specifications

Specification	Status	Comment
StreamsThe definition of 'getReader()' 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
`getReader` Experimental'	Chrome Full support 43	Edge Full support 14	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 Full support 30	Safari Full support 10.1	WebView Android Full support 43	Chrome Android Full support 43	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 Full support 30	Safari iOS Full support 10.3	Samsung Internet Android Full support 4.0

Legend

Full support: Full support
No support: No support
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.

ReadableStream.getReader() by Mozilla Contributors is licensed under CC-BY-SA 2.5.