The cancel()
method of the ReadableStream
interface cancels the associated stream. The supplied reason
parameter will be given to the underlying source, which may or may not use it.
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
var promise = readableStream.cancel(reason);
Parameters
- reason
- A
DOMString
providing a human-readable reason for the cancellation.
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.
var searchTerm = "service workers"; // Chars to show either side of the result in the match var contextBefore = 30; var contextAfter = 30; var caseInsensitive = true; var url = 'https://html.spec.whatwg.org/'; console.log(`Searching '${url}' for '${searchTerm}'`); fetch(url).then(response => { console.log('Received headers'); var decoder = new TextDecoder(); var reader = response.body.getReader(); var toMatch = caseInsensitive ? searchTerm.toLowerCase() : searchTerm; var bufferSize = Math.max(toMatch.length - 1, contextBefore); var bytesReceived = 0; var buffer = ''; var 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 contextβ¦'); } // keep reading return reader.read().then(process); }); }).catch(err => { console.log("Something went wrong. See devtools for details. Does the response lack CORS headers?"); throw err; });
Specifications
Specification | Status | Comment |
---|---|---|
Streams The definition of 'cancel()' in that specification. |
Living Standard | Initial definition. |
Browser compatibility
The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.
Desktop | Mobile | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
cancel | Chrome Full support 43 | Edge Full support 14 | Firefox
Full support
65
| IE No support No | Opera Full support 30 | Safari Full support 10.1 | WebView Android Full support 43 | Chrome Android Full support 43 | Firefox Android
Full support
65
| Opera Android Full support 30 | Safari iOS Full support 10.3 | Samsung Internet Android Full support 4.0 |
Legend
- Full support
- Full support
- No support
- No support
- Experimental. Expect behavior to change in the future.
- Experimental. Expect behavior to change in the future.
- User must explicitly enable this feature.
- User must explicitly enable this feature.