files/en-us/web/api/readablestreamdefaultreader/cancel/index.md
{{APIRef("Streams")}}{{AvailableInWorkers}}
The cancel() method of the
{{domxref("ReadableStreamDefaultReader")}} interface returns a {{jsxref("Promise")}} that resolves when the stream is canceled. Calling this method signals a loss of interest in the stream by a consumer.
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 {{domxref("ReadableStreamDefaultController.close()")}}.
[!NOTE] If the reader is active, the
cancel()method behaves the same as that for the associated stream ({{domxref("ReadableStream.cancel()")}}).
cancel()
cancel(reason)
reason {{optional_inline}}
A {{jsxref("Promise")}}, which fulfills with the value given in the reason
parameter.
ReadableStreamDefaultReader, or the stream
has no owner.In the following simple example, a previously-created custom
ReadableStream is read using a {{domxref("ReadableStreamDefaultReader")}}
created using getReader(). (this code is based on our Simple random stream example). 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.
When the stream is done (if (done)), we run reader.cancel()
to cancel the stream, signalling that we don't need to use it any more.
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");
reader.cancel();
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}}
{{Compat}}