a Web Storage example.

webStorage.js

'use strict';

const MyStorage = function(app) {
  this.app = app;
  this.storage = localStorage;
  this.data = JSON.parse(this.storage[this.app] || '{}');
};

MyStorage.prototype = {
  getItem: function(key) {
    return this.data[key];
  },
  setItem: function(key, value) {
    this.data[key] = value;
    return this.data[key];
  },
  removeItem: function(key) {
    let obj = JSON.parse(this.storage[this.app]);
    delete obj[key];
    return obj;
  },
  save: function(obj) {
    this.storage[this.app] = JSON.stringify(obj || this.data);
  }
};

window.addEventListener('DOMContentLoaded', function() {
  const storage = new MyStorage('SampleApp');
  const memo = document.getElementById('memo');
  const clear = document.getElementById('clear');

  memo.value = storage.getItem('wsMemo') || '';

  memo.addEventListener('keyup', function() {
    storage.setItem('wsMemo', memo.value);
    storage.save();
  });

  clear.addEventListener('click', function() {
    memo.value = null;
    storage.save(storage.removeItem('wsMemo'));
  });
});