ReadableStream: cancel() method

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 ReadableStreamDefaultController.close().

Syntax

cancel()
cancel(reason)

Parameters

reason Optional: A human-readable reason for the cancellation. The underlying source may or may not use it.

Return value

A Promise, which fulfills with the value given in the reason parameter.

Exceptions

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

Examples

In Jake Archibald's cancelling a fetch 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.

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/";

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

fetch(url)
  .then((response) => {
    console.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) {
        console.log("Failed to find match");
        return;
      }

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

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

Specifications

Specification
Streams Standard # ref-for-rs-cancel③

Browser compatibility

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