hydrate-from-external-sources.js

hydrate() {
  if (this.hydrated) {
    return Promise.resolve()
  }

  if (this._hydrating) {
    return this._hydrating
  }

  // This assigns the promise SYNCHRONOUSLY within the method call, which
  // is important for re-entrancy. If we await()ed anything above this point,
  // we could have a race.
  return this._hydrating = (async () => {
    // NOW we can do async work.
    await whatever()
    this.hydrated = true;
  })().finally(() => {
    // After finishing, clear out the promise.
    this._hydrating = null
  })
}