degenerate.js -- promise fulfilling yield handler for generator functions

degenerate.js

function degenerate (generator) {
  return function () {
    var iterator = generator.apply(this, arguments);

    return new Promise(function fn (resolve, reject) {
      return pass(void 0);

      function pass(value) {
        coop.call(iterator.next, iterator, value, resolve, reject, pass, fail);
      }

      function fail (err) {
        coop.call(iterator.throw, iterator, err, resolve, reject, pass, fail);
      }
    });
  }
}

function coop (iterator, value, resolve, reject, pass, fail) {
  var resume = "function" === typeof this ? this : iterator.next;
  var result;

  do {
    try {
      if (!result) {
        result = resume.call(iterator, value);
      } else {
        result = iterator.next(result.value);
      }
    } catch (err) {
      if (reject) {
        return reject(err);
      } else {
        throw err;
      }
    }

    if (result.done) {
      return resolve ? resolve(result.value) : result.value;
    } else if (!result.value) {
      continue;
    } else if ("function" === typeof result.value.then) {
      return result.value.then(pass, fail);
    } else {
      continue;
    }
  } while (true);
}