Read a ND-JSON stream from the browser or from nodejs

browser-ndjson-stream-reader.js

/* FOR THE BROWSER
Utility function to read a ND-JSON HTTP stream.
`processLine` is a function taking a JSON object. It will be called with each element of the stream.
`response` is the result of a `fetch` request.
See usage example in the next file.
*/

const readStream = processLine => response => {
  const stream = response.body.getReader();
  const matcher = /\r?\n/;
  const decoder = new TextDecoder();
  let buf = '';

  const loop = () =>
    stream.read().then(({ done, value }) => {
      if (done) {
        if (buf.length > 0) processLine(JSON.parse(buf));
      } else {
        const chunk = decoder.decode(value, {
          stream: true
        });
        buf += chunk;

        const parts = buf.split(matcher);
        buf = parts.pop();
        for (const i of parts.filter(p => p)) processLine(JSON.parse(i));
        return loop();
      }
    });

  return loop();
}

nodejs-ndjson-stream-reader.js

/* FOR NODEJS
Utility function to read a ND-JSON HTTP stream.
`processLine` is a function taking a JSON object. It will be called with each element of the stream.
`response` is the result of a `fetch` request.
See usage example in the next file.
*/
const readStream = processLine => response => {
  const matcher = /\r?\n/;
  const decoder = new TextDecoder();
  let buf = '';
  return new Promise((resolve, fail) => {
    response.body.on('data', v => {
      const chunk = decoder.decode(v, { stream: true });
      buf += chunk;

      const parts = buf.split(matcher);
      buf = parts.pop();
      for (const i of parts.filter(p => p)) processLine(JSON.parse(i));
    });
    response.body.on('end', () => {
      if (buf.length > 0) processLine(JSON.parse(buf));
      resolve();
    });
    response.body.on('error', fail);
  });
};

usage.js

/* From browser or nodejs alike */

const stream = fetch('https://lichess.org/api/tv/feed');
// or any other ND-JSON endpoint such as:
// const stream = fetch('https://lichess.org/api/games/user/neio',{headers:{Accept:'application/x-ndjson'}});

const onMessage = obj => console.log(obj);
const onComplete = () => console.log('The stream has completed');

stream
  .then(readStream(onMessage))
  .then(onComplete);