ReadableStreamDefaultReader.ReadableStreamDefaultReader()

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This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

The ReadableStreamDefaultReader() constructor creates and returns a ReadableStreamDefaultReader() object instance.

Note: You generally wouldn't use this constructor manually; instead, you'd use the ReadableStream.getReader() method.

Syntax

var readableStreamReader = new ReadableStreamDefaultReader(stream);

Parameters

stream
The ReadableStream to be read.

Return value

An instance of the ReadableStreamDefaultReader object.

Exceptions

TypeError
The supplied stream parameter is not a ReadableStream, or it is already locked for reading by another reader.

Examples

In the following simple example, a previously-created custom ReadableStream is read using a ReadableStreamDefaultReader created using getReader(). (see our 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 = 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);
  });
}

Specifications

Specification Status Comment
Streams
The definition of 'ReadableStreamDefaultReader()' in that specification.
Living Standard Initial definition.

Browser compatibility

No compatibility data found. Please contribute data for "path.to.feature.NameOfTheConstructor" (depth: 1) to the MDN compatibility data repository.

Document Tags and Contributors

 Contributors to this page: chrisdavidmills
 Last updated by: chrisdavidmills,