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.


var readableStreamDefaultReader = new ReadableStreamDefaultReader(stream);


The ReadableStream to be read.

Return value

An instance of the ReadableStreamDefaultReader object.


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


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 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;

    // 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;

    result += chunk;

    // Read some more, and call this function again


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

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