Simple JS Cache Persistence

cache.js

window.cache = (function () {

  var _data = {};

  var _db =  openDatabase(
    'cache.db',
    '1.0',
    'app cache database',
    5 * 1024 * 1024
  )

  function executeQuery (query, values, tx) {
    values = values || []
    return new Promise (resolve, reject) {
      var exec = function exec (tx) {
        tx.executeSql(
          query,
          values,
          resolve,
          reject
        )
      }

      if (tx) {
        exec(tx)
      } else {
        _db.transaction(function (tx) {
          exec(tx)
        }, reject)
      }
    }
  }

  function _save (key) {
    if (_data[key] != null) {
      return executeQuery(
        'INSERT OR REPLACE INTO cache (id, val) VALUES (?, ?);',
        [ key, JSON.stringify(_data[key]) ]
      )
    } else {
      return Promise.resolve()
    }
  }
  
  function _remove (key) {
    return executeQuery('DELETE FROM cache WHERE id = ?;', [ key ])
  }

  function get (key) {
    return _data[key];
  }

  function set (key, val) {
    return _save(key).then(function() {
      _data[key] = val;
    })
  }

  function del (key) {
    return _remove(key).then(function () {
      delete _data[key]
    })
  }

  function loadDatabase () {
    return executeQuery(
      'CREATE TABLE IF NOT EXISTS cache (id TEXT PRIMARY KEY, val TEXT);'
    )
      .then(function(tx) {
        return executeQuery(
          'CREATE TABLE IF NOT EXISTS cache (id TEXT PRIMARY KEY, val TEXT);',
          [],
          tx
        )
      })
      .then(function(tx, res) {
        for (var i = 0, l = res.rows.length; i < l; i++) {
          _data[res.rows.item(i).id] = JSON.parse(res.rows.item(i).val)
        }
      })
  }

  return {
    get : get,
    set : set,
    del : del,
    loadDatabase : loadDatabase
  }

}())