var searchTerm = "service workers";

var contextBefore = 30;

var contextAfter = 30;

var caseInsensitive = true;

​

console.log('Fetching');

​

fetch('https://html.spec.whatwg.org/').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, 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});

​

    if (matchFoundAt === -1) {

      if (caseInsensitive) {

        matchFoundAt = (caseInsensitive ? buffer.toLowerCase() : buffer).indexOf(toMatch);

      }

    }

​

    if (matchFoundAt === -1) {

      buffer = buffer.slice(0, -bufferSize);

    }

    else if (buffer.slice(matchFoundAt + toMatch.length).length >= contextAfter) {

      console.log(buffer.slice(Math.max(0, matchFoundAt - contextBefore), matchFoundAt + toMatch.length + contextAfter));

      reader.cancel();

      return;

    }

    else {

      console.log('Found match, but need more context…');

    }

​

    // keep reading

    return reader.read().then(process);

  });

});