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.

{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.

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

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.

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 = 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);
  });
}