async has (cacheName) {
    webidl.brandCheck(this, CacheStorage)

    const prefix = 'CacheStorage.has'
    webidl.argumentLengthCheck(arguments, 1, prefix)

    cacheName = webidl.converters.DOMString(cacheName, prefix, 'cacheName')

    // 2.1.1
    // 2.2
    return this.#caches.has(cacheName)
  }
@see https://w3c.github.io/ServiceWorker/#cache-storage-has
@param {string} cacheName
@returns {Promise<boolean>}