Simple key-object cache with Greasemonkey and local storage options

cache.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' })