React.lazy

const Unitialized = -1;
const Pending = 0;
const Resolved = 1;
const Rejected = 2;

function lazy(ctor) {
  // hold component state in closure
  const payload = {
    _status: Unitialized,
    _result: ctor
  }

  return {
    $$typeof: REACT_LAZY_TYPE,
    _payload: payload,
    _init: lazyInitializer
  }
}

function lazyInitalizer(payload) {
  // import when first initialize
  if (payload._status === Uninitialized) {
    const thenable = payload._result();
    // Transition to the Pending state.
    payload._status = Pending,
    payload._result = thenable

    thenable.then(
      // save result to payload
      moduleObject => {
        if (payload._status === Pending) {
          // Transition to the Resolved state.
          payload._status = Resolved
          payload._result = moduleObject.default
        }
      },
      error => {
        if (payload._status === Pending) {
          // Transition to the Rejected state.
          payload._status = Rejected;
          payload._result = error;
        }
      },
    );
  } else if (payload._status === Resolved) {
    // return result when resolved.
    return payload._result;
  } else {
    // throw Promise or Error when status is Pending or Rejected.
    throw payload._result;
  }
}