something like stream.readline(), returning a promise.

scannable-stream.js

import {Writable} from "stream";

// scanner returns remaining string if succeeded, null/undefined otherwise
class ScanningState {
  constructor(buf, scanner) {
    this.buf = buf;
    this.scanner = scanner;
  }
  scan() {
    throw new Error("Another scan in progress");
  }
  data(chunk, _continue) {
    var buf = this.buf + chunk;
    var remaining = this.scanner(buf);
    if (typeof remaining === "string") {
      return new PausedBlockState(remaining, _continue);
    }
    _continue();
    return new ScanningState(buf, this.scanner);
  }
}

class PausedNoBlockState {
  constructor(buf) {
    this.buf = buf;
  }
  scan(scanner) {
    var remaining = scanner(this.buf);
    if (typeof remaining === "string")
      return new PausedNoBlockState(remaining);
    return new ScanningState(this.buf, scanner);
  }
  data(chunk, _continue) {
    return new PausedBlockState(this.buf + chunk, _continue);
  }
}

class PausedBlockState {
  constructor(buf, _continue) {
    this.buf = buf;
    this.continue = _continue;
  }
  scan(scanner) {
    var remaining = scanner(this.buf);
    if (typeof remaining === "string")
      return new PausedBlockState(remaining, this.continue);
    this.continue();
    return new ScanningState(this.buf, scanner);
  }
  data() {
    throw new Error("Are you really using node streams?");
  }
}

export default class ScannableStream extends Writable {
  constructor() {
    super({
      decodeStrings: false,
      objectMode: false
    });
    this._state = new PausedNoBlockState("");
  }

  _write(chunk, encoding, _continue) {
    this._state = this._state.data(chunk, _continue);
  }

  scan(sep) {
    return new Promise(resolve => {
      if (typeof sep === "string")
        this._state = this._state.scan(scanByString(sep, resolve));
      else this._state = this._state.scan(scanByRegex(sep, resolve));
    });
  }
}

function scanByString(sep, resolve) {
  return function stringScanner(buf) {
    var iSep = buf.indexOf(sep);
    if (iSep >= 0) {
      resolve(buf.substring(0, iSep));
      return buf.substr(iSep + sep.length);
    }
  };
}

function scanByRegex(sep, resolve) {
  return function regexScanner(buf) {
    var match = sep.exec(buf);
    if (match) {
      resolve(buf.substring(0, match.index));
      return buf.substr(match.index + match[0].length);
    }
  };
}