ReadableStream: cancel() method

Available in workers

The cancel() method of the ReadableStream interface returns a Promise that resolves when the stream is canceled.

Cancel is used when you’ve completely finished with the stream and don’t need any more data from it, even if there are chunks enqueued waiting to be read. That data is lost after cancel is called, and the stream is not readable any more. To read those chunks still and not completely get rid of the stream, you’d use close().

Syntax

cancel()
cancel(reason)

Parameters

• reason Optional
  • : A human-readable reason for the cancellation. This is passed to the underlying source, which may or may not use it.

Return value

A Promise, which fulfills with undefined value.

Exceptions

• TypeError
  • : The stream you are trying to cancel is not a ReadableStream, or it is locked.

Examples

In the following example, a stream is used to fetch the WHATWG HTML spec chunk by chunk; each chunk is searched for the string “service workers”. When the search terms is found, cancel() is used to cancel the stream — the job is finished so it is no longer needed.

<pre id="output"></pre>

const output = document.getElementById("output");

function log(text) {
  output.textContent += `${text}\n`;
}

const searchTerm = "service workers";
// Chars to show either side of the result in the match
const contextBefore = 30;
const contextAfter = 30;
const caseInsensitive = true;
const url = "https://html.spec.whatwg.org/";

log(`Searching '${url}' for '${searchTerm}'`);

fetch(url)
  .then((response) => {
    log("Received headers");

    const decoder = new TextDecoder();
    const reader = response.body.getReader();
    const toMatch = caseInsensitive ? searchTerm.toLowerCase() : searchTerm;
    const bufferSize = Math.max(toMatch.length - 1, contextBefore);

    let bytesReceived = 0;
    let buffer = "";
    let matchFoundAt = -1;

    return reader.read().then(function process(result) {
      if (result.done) {
        log("Failed to find match");
        return;
      }

      bytesReceived += result.value.length;
      log(`Received ${bytesReceived} bytes of data so far`);

      buffer += decoder.decode(result.value, { stream: true });

      // already found match & just context-gathering?
      if (matchFoundAt === -1) {
        matchFoundAt = (
          caseInsensitive ? buffer.toLowerCase() : buffer
        ).indexOf(toMatch);
      }

      if (matchFoundAt === -1) {
        buffer = buffer.slice(-bufferSize);
      } else if (
        buffer.slice(matchFoundAt + toMatch.length).length >= contextAfter
      ) {
        log("Here's the match:");
        log(
          buffer.slice(
            Math.max(0, matchFoundAt - contextBefore),
            matchFoundAt + toMatch.length + contextAfter,
          ),
        );
        log("Cancelling fetch");
        reader.cancel();
        return;
      } else {
        log("Found match, but need more context…");
      }

      // keep reading
      return reader.read().then(process);
    });
  })
  .catch((err) => {
    log(
      "Something went wrong. See devtools for details. Does the response lack CORS headers?",
    );
    throw err;
  });

Specifications

Browser compatibility

See also

• ReadableStream() constructor
• Using readable streams