ReadableStream.cancel()

var promise = readableStream.cancel(reason);

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 context…');
    }

    // 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;
});