ReadableStreamDefaultReader: read() method

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since January 2020.

Note: This feature is available in Web Workers.

The read() method of the ReadableStreamDefaultReader interface returns a Promise providing access to the next chunk in the stream's internal queue.

Syntax

read()

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, the stream has no owner, or ReadableStreamDefaultReader.releaseLock() is called (when there's a pending read request).

Examples

Example 1 - simple example

This example shows the basic API usage, but doesn't try to deal with complications like stream chunks not ending on line boundaries for example.

In this example, stream is a previously-created custom ReadableStream. It 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 as an array of UTF-8 bytes, 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.

js
function fetchStream() {
  const reader = stream.getReader();
  let charsReceived = 0;

  // read() returns a promise that fulfills
  // 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);
  });
}

Example 2 - handling text line by line

This example shows how you might fetch a text file and handle it as a stream of text lines. It deals with stream chunks not ending on line boundaries, and with converting from Uint8Array to strings.

js
async function* makeTextFileLineIterator(fileURL) {
  const utf8Decoder = new TextDecoder("utf-8");
  let response = await fetch(fileURL);
  let reader = response.body.getReader();
  let { value: chunk, done: readerDone } = await reader.read();
  chunk = chunk ? utf8Decoder.decode(chunk, { stream: true }) : "";

  let re = /\r?\n/g;
  let startIndex = 0;

  for (;;) {
    let result = re.exec(chunk);
    if (!result) {
      if (readerDone) {
        break;
      }
      let remainder = chunk.substring(startIndex);
      ({ value: chunk, done: readerDone } = await reader.read());
      chunk =
        remainder + (chunk ? utf8Decoder.decode(chunk, { stream: true }) : "");
      startIndex = re.lastIndex = 0;
      continue;
    }
    yield chunk.substring(startIndex, result.index);
    startIndex = re.lastIndex;
  }
  if (startIndex < chunk.length) {
    // last line didn't end in a newline char
    yield chunk.substring(startIndex);
  }
}

for await (let line of makeTextFileLineIterator(urlOfFile)) {
  processLine(line);
}