```js 'use strict';

// --------------------------------------------------------------------

class Cache extends Map { constructor(key) { super() this._key = key this.load() }

set(k, v) { if (!this.has(k) || v !== this.get(k)) { super.set(k, v) this.save() } }

delete(k) { if (super.has(k)) { super.delete(k) this.save() } }

clear() { if (super.size() > 0) { super.clear() this.save() } }

json() { let obj = Object.create(null)

for (let k of this.keys()) {
  obj[k] = this.get(k)
}

return obj

}

save() { const data = this.json() this._save(this._key, data) this.dump({ data, event: 'SAVE' }) }

load(s) { const data = this._load(this._key)

if (typeof data !== 'object') {
  return
}

super.clear()

for (let k of Object.keys(data)) {
  super.set(k, data[k])
}

this.dump({ data, event: 'LOAD' })

}

_save(key, data) {} _load(key) {}

edit() { let res = window.prompt('Edit cached package URLs', JSON.stringify(this.json(), null, 2))

if (res !== null) {
  try {
    const data = JSON.parse(res)

    super.clear()

    for (let k of Object.keys(data)) {
      super.set(k, data[k])
    }

    this.save()
  }
  catch (ex) {
    console.warn('Failed to update cache data: %s %o', ex.toString(), ex)
  }
}

}

toString() { return ${this.constructor.name}<${this._key}>: keys=[ ${this.keys().sort().join(', ')} ] }

dump({ data, event }) { console.group(${this.constructor.name}<${this._key}>: ${event || 'STATE'}:) console.info(JSON.stringify(data || this.json(), null, 2)) console.groupEnd() } }

// --------------------------------------------------------------------

class GMCache extends Cache { _save(key, data) { GM_setValue(key, JSON.stringify(data || {})) }

_load(key) { return JSON.parse(GM_getValue(key) || '{}') } }

// --------------------------------------------------------------------

class StorageCache extends Cache { constructor(key, session) { super(key)

this.storage = session ? window.sessionStorage : window.localStorage
this.load()

}

_save(key, data) { this.storage.setItem(key, JSON.stringify(data || {})) }

_load(key) { if (this.storage) { return JSON.parse(this.storage.getItem(key) || '{}') } } }

// --------------------------------------------------------------------

let c = new StorageCache('test-data') // c.set('jshint-summary', { name: 'jshint-summary', homepage: 'https://github.com/spiralx/jshint-summary' }) ```