Example of how to use yield* to combine generators

yield-star.js

// Our incremental JSON parser (simplified only to use strings and arrays)
// while iterator result != done keep using .next(moreSrc) or .next(null) if you are done providing data
// while the iterator is not done it will return the state of the iterator

// the iterator is seeking more data to get a valid value
// call .next(str) to provide it more data
var NEED_MORE_DATA = 0;

// the iterator has a valid result and will return a result if you provide .next(null)
// this is not automatic, due to numbers and potentially invalid trailing characters after an expected end
var HAVE_RESULT = 1;
function* parseJSON(src) {
  return (yield* parseJSONValue(src, false)).result;
}

function removeStartingSpaces(src) {
  return src.replace(/^\s+/, '');
}
function checkForUnexpectedEnd(src) {
  if (src == null) {
    throw new Error('unexpected end of source');
  }
  return src;
}
function needToCheckTrailingSource(src) {
  if (src == null) {
    return false;
  }
  var invalidCharacter = /\S/.exec(src);
  if (invalidCharacter) {
    throw new Error('Found an invalid character after string ' + invalidCharacter[0]);
  }
}
// our "internal" method, used to grab all of the types of data
function* parseJSONValue(src, stop_on_result) {
  while (src != null) {
    src = removeStartingSpaces(src);
    if (src.length) switch (src[0]) {
      case '"':
        var value = yield* parseJSONString(src, stop_on_result);
        return value;
      case '[':
        var value = yield* parseJSONArray(src, stop_on_result);
        return value;
      default:
        throw new Error('Unexpected character while expecting value');
    }
    src = yield checkForUnexpectedEnd(src);
  }
  checkForUnexpectedEnd(src);
}
function* parseJSONString(src, stop_on_result) {
  // skip starting spaces
  while (!src.length) {
    src = removeStartingSpaces(checkForUnexpectedEnd(yield NEED_MORE_DATA));
  }
  if (src[0] !== '"') {
    throw new Error('Unexpected character while expecting string ' + src[i]);
  }
  var result = '';
  var escaped = false;
  src = src.slice(1);
  parse_loop:
  while (true) {
    for (var i = 0; i < src.length; i++) {
      if (escaped) {
        // ideally we would add some checks here, unicode, but this is just an example
        result += src[i];
      }
      else if (src[i] == '\\') {
        escaped = true;
      }
      else if (src[i] == '"') {
        src = src.slice(i+1);
        break parse_loop;
      }
      else {
        result += src[i];
      }
    }
    src = checkForUnexpectedEnd(yield NEED_MORE_DATA);
  }
  if (!stop_on_result) {
    while (needToCheckTrailingSource(yield HAVE_RESULT));
  }
  return { result: result, unused: src };
}
function* parseJSONArray(src, stop_on_result) {
  // skip starting spaces
  while (!src.length) {
    src = removeStartingSpaces(checkForUnexpectedEnd(yield NEED_MORE_DATA));
  }
  if (src[0] !== '[') {
    throw new Error('Unexpected character while expecting array ' + src[i]);
  }
  var result = [];
  // remove opening bracket and spaces
  src = removeStartingSpaces(src.slice(1));
  // skip spaces
  while (!src.length) {
    src = removeStartingSpaces(checkForUnexpectedEnd(yield NEED_MORE_DATA));
  }
  if (src[0] != ']') while (true) {
    // we want to grab a value from our iteration, but halt on getting a value
    var values = yield* parseJSONValue(src, true);
    result.push(values.result);
    // keep processing data if we have stuff left over
    src = values.unused;
    // request more data, just skip starting spaces
    while (!src.length) {
      src = removeStartingSpaces(checkForUnexpectedEnd(yield NEED_MORE_DATA).replace(/^\s+/, ''));
    }
    if (src[0] == ']') {
      break;
    }
    if (src[0] != ',') throw new Error('Found an invalid character while expecting end of array or comma');
    // remove comma
    src = src.slice(1);
  }
  // remove trailing backet
  src = src.slice(1);
  if (!stop_on_result) {
    while (needToCheckTrailingSource(yield HAVE_RESULT));
  }
  return { result: result, unused: src };
}
g=parseJSON('[[""],"test"]');
g.next();