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

Promise<result> readableStreamDefaultReaderInstance.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

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

Browser compatibility

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