DOM Storage guide

সারাংশ

ওয়েব এপ্লিকেশন ১.0  স্পেসিফেকেশন এ প্রথমবারের মত  স্টোরেজ সম্পর্কিত বৈশিষ্ট্যসমূহ এর সেটকে ডোম স্টোরেজ নাম দেওয়া হয় এবং এখন এটি এর নিজস্ব  ডব্লিউ থ্রি সি ওয়েব স্টোরেজ (W3C Web Storage)  স্পেসিফিকেশন লাভ করেছে।  ডোম স্টোরেজ ডিজাইন করা হয়েছে ইনফরমেশন জমা রাখার কুকির সাপেক্ষে  আরো বড়,আরো সুরক্ষিত এবং সহজে ব্যবহারযোগ্য প্রতিস্থাপক  হিসেবে। এটি প্রথমবারের মত  ফায়ারফক্স ২ এবং সাফারি ৪  এ প্রণীত হয়।

Description

The DOM Storage mechanism is a means through which string key/value pairs can be securely stored and later retrieved for use. The goal of this addition is to provide a comprehensive means through which interactive applications can be built (including advanced abilities, such as being able to work "offline" for extended periods of time).

Mozilla-based browsers, Internet Explorer 8+, Safari 4+, and Chrome all provide a working implementation of the DOM Storage specification. (In case you need this functionality cross-browser and you need to cater for older versions of IE, it may be useful to note that IE also has a similar legacy feature called "userData behavior" that pre-dates the addition of DOM Storage to IE in IE8.)

DOM Storage is useful because no good browser-only methods exist for persisting reasonable amounts of data for any period of time. Browser cookies have limited capacity and provide no support for organizing persisted data, and other methods (such as Flash Local Storage) require an external plugin.

One of the first public applications to make use of the new DOM Storage functionality (in addition to Internet Explorer's userData Behavior) was halfnote (a note-taking application) written by Aaron Boodman. In his application, Aaron simultaneously saved notes back to a server (when Internet connectivity was available) and a local data store. This allowed the user to safely write backed-up notes even with sporadic Internet connectivity.

While the concept, and implementation, presented in halfnote were comparatively simple, halfnote's creation shows the possibility for a new breed of web applications that are usable both online and offline.

Reference

The following are global objects that exist as properties of every window object. This means that they can be accessed as sessionStorage or window.sessionStorage. (This is important because you can then use IFrames to store, or access, additional data beyond what is immediately included in your page.)

Storage

This is a constructor (Storage) for all Storage instances (sessionStorage and globalStorage[location.hostname]). Setting Storage.prototype.removeKey = function(key){ this.removeItem(this.key(key)) } would be accessed as localStorage.removeKey and sessionStorage.removeKey.

globalStorage items are not an instance of Storage, but instead are an instance of StorageObsolete.

Storage is defined by the WhatWG Storage Interface as this:

interface Storage {
  readonly attribute unsigned long length;
  [IndexGetter] DOMString key(in unsigned long index);
  [NameGetter] DOMString getItem(in DOMString key);
  [NameSetter] void setItem(in DOMString key, in DOMString data);
  [NameDeleter] void removeItem(in DOMString key);
  void clear();
};

sessionStorage

This is a global object (sessionStorage) that maintains a storage area that's available for the duration of the page session. A page session lasts for as long as the browser is open and survives over page reloads and restores. Opening a page in a new tab or window will cause a new session to be initiated.

// Save data to the current session's store
sessionStorage.setItem("username", "John");

// Access some stored data
alert( "username = " + sessionStorage.getItem("username"));

The sessionStorage object is most useful for hanging on to temporary data that should be saved and restored if the browser is accidentally refreshed.

Examples:

Autosave the contents of a text field, and if the browser is accidentally refreshed, restore the text field contents so that no writing is lost.

 // Get the text field that we're going to track
 var field = document.getElementById("field");
 
 // See if we have an autosave value
 // (this will only happen if the page is accidentally refreshed)
 if ( sessionStorage.getItem("autosave")) {
    // Restore the contents of the text field
    field.value = sessionStorage.getItem("autosave");
 }
 
 // Check the contents of the text field every second
 setInterval(function(){
    // And save the results into the session storage object
    sessionStorage.setItem("autosave", field.value);
 }, 1000);

More information:

• sessionStorage specification

localStorage

localStorage is the same as sessionStorage with same same-origin rules applied but it is persistent. localStorage was introduced in Firefox 3.5.

Compatibility

Storage objects are a recent addition to the standard. As such they may not be present in all browsers. You can work around this by inserting one of the following two codes at the beginning of your scripts, allowing use of localStorage object in implementations which do not natively support it.

This algorithm is an exact imitation of the localStorage object, but makes use of cookies.

if (!window.localStorage) {
  Object.defineProperty(window, "localStorage", new (function () {
    var aKeys = [], oStorage = {};
    Object.defineProperty(oStorage, "getItem", {
      value: function (sKey) { return sKey ? this[sKey] : null; },
      writable: false,
      configurable: false,
      enumerable: false
    });
    Object.defineProperty(oStorage, "key", {
      value: function (nKeyId) { return aKeys[nKeyId]; },
      writable: false,
      configurable: false,
      enumerable: false
    });
    Object.defineProperty(oStorage, "setItem", {
      value: function (sKey, sValue) {
        if(!sKey) { return; }
        document.cookie = escape(sKey) + "=" + escape(sValue) + "; expires=Tue, 19 Jan 2038 03:14:07 GMT; path=/";
      },
      writable: false,
      configurable: false,
      enumerable: false
    });
    Object.defineProperty(oStorage, "length", {
      get: function () { return aKeys.length; },
      configurable: false,
      enumerable: false
    });
    Object.defineProperty(oStorage, "removeItem", {
      value: function (sKey) {
        if(!sKey) { return; }
        document.cookie = escape(sKey) + "=; expires=Thu, 01 Jan 1970 00:00:00 GMT; path=/";
      },
      writable: false,
      configurable: false,
      enumerable: false
    });
    this.get = function () {
      var iThisIndx;
      for (var sKey in oStorage) {
        iThisIndx = aKeys.indexOf(sKey);
        if (iThisIndx === -1) { oStorage.setItem(sKey, oStorage[sKey]); }
        else { aKeys.splice(iThisIndx, 1); }
        delete oStorage[sKey];
      }
      for (aKeys; aKeys.length > 0; aKeys.splice(0, 1)) { oStorage.removeItem(aKeys[0]); }
      for (var aCouple, iKey, nIdx = 0, aCouples = document.cookie.split(/\s*;\s*/); nIdx < aCouples.length; nIdx++) {
        aCouple = aCouples[nIdx].split(/\s*=\s*/);
        if (aCouple.length > 1) {
          oStorage[iKey = unescape(aCouple[0])] = unescape(aCouple[1]);
          aKeys.push(iKey);
        }
      }
      return oStorage;
    };
    this.configurable = false;
    this.enumerable = true;
  })());
}

Here is another, less exact, imitation of the localStorage object. It is simpler than the previous one, but it is compatible with old browsers, like Internet Explorer < 8 (tested and working even in Internet Explorer 6). It also makes use of cookies.

if (!window.localStorage) {
  window.localStorage = {
    getItem: function (sKey) {
      if (!sKey || !this.hasOwnProperty(sKey)) { return null; }
      return unescape(document.cookie.replace(new RegExp("(?:^|.*;\\s*)" + escape(sKey).replace(/[\-\.\+\*]/g, "\\$&") + "\\s*\\=\\s*((?:[^;](?!;))*[^;]?).*"), "$1"));
    },
    key: function (nKeyId) {
      return unescape(document.cookie.replace(/\s*\=(?:.(?!;))*$/, "").split(/\s*\=(?:[^;](?!;))*[^;]?;\s*/)[nKeyId]);
    },
    setItem: function (sKey, sValue) {
      if(!sKey) { return; }
      document.cookie = escape(sKey) + "=" + escape(sValue) + "; expires=Tue, 19 Jan 2038 03:14:07 GMT; path=/";
      this.length = document.cookie.match(/\=/g).length;
    },
    length: 0,
    removeItem: function (sKey) {
      if (!sKey || !this.hasOwnProperty(sKey)) { return; }
      document.cookie = escape(sKey) + "=; expires=Thu, 01 Jan 1970 00:00:00 GMT; path=/";
      this.length--;
    },
    hasOwnProperty: function (sKey) {
      return (new RegExp("(?:^|;\\s*)" + escape(sKey).replace(/[\-\.\+\*]/g, "\\$&") + "\\s*\\=")).test(document.cookie);
    }
  };
  window.localStorage.length = (document.cookie.match(/\=/g) || window.localStorage).length;
}

Compatibility and relation with globalStorage

localStorage is also the same as globalStorage[location.hostname], with the exception of being scoped to an HTML5 origin (scheme + hostname + non-standard port) and localStorage being an instance of Storage as opposed to globalStorage[location.hostname] being an instance of StorageObsolete which is covered below. For example, https://example.com is not able to access the same localStorage object as https://example.com but they can access the same globalStorage item. localStorage is a standard interface while globalStorage is non-standard so you shouldn't rely on these.

Please note that setting a property on globalStorage[location.hostname] does not set it on localStorage and extending Storage.prototype does not affect globalStorage items; only extending StorageObsolete.prototype does.

globalStorage

globalStorage is obsolete since Gecko 1.9.1 (Firefox 3.5) and unsupported since Gecko 13 (Firefox 13). Just use localStorage instead. This proposed addition to HTML5 has been removed from the HTML5 specification in favor of localStorage, which is implemented in Firefox 3.5. This is a global object (globalStorage) that maintains multiple private storage areas that can be used to hold data over a long period of time (e.g., over multiple pages and browser sessions).

// Save data that only scripts on the mozilla.org domain can access
globalStorage['mozilla.org'].setItem("snippet", "<b>Hello</b>, how are you?");

Specifically, the globalStorage object provides access to a number of different storage objects into which data can be stored. For example, if we were to build a web page that used globalStorage on this domain (developer.mozilla.org) we'd have the following storage object available to us:

• globalStorage['developer.mozilla.org'] - All web pages within the developer.mozilla.org sub-domain can both read and write data to this storage object.

Examples:

All of these examples require that you have a script inserted (with each of the following code) in every page that you want to see the result on.

Remember a user's username for the particular sub-domain that is being visited:

 globalStorage['developer.mozilla.org'].setItem("username", "John");

Keep track of the number of times that a user visits all pages of your domain:

 // parseInt must be used since all data is stored as a string
 globalStorage['mozilla.org'].setItem("visits", parseInt(globalStorage['mozilla.org'].getItem("visits") || 0 ) + 1);

Storage location and clearing the data

In Firefox the DOM storage data is stored in the webappsstore.sqlite file in the profile folder (there's also chromeappsstore.sqlite file used to store browser's own data, notably for the start page - about:home, but potentially for other internal pages with "about:" URLs).

• DOM Storage can be cleared via "Tools -> Clear Recent History -> Cookies" when Time range is "Everything" (via nsICookieManager::removeAll)
  • But not when another time range is specified: (bug 527667)
  • Does not show up in Tools -> Options -> Privacy -> Remove individual cookies (bug 506692)
• DOM Storage is not cleared via Tools -> Options -> Advanced -> Network -> Offline data -> Clear Now.
• Doesn't show up in the "Tools -> Options -> Advanced -> Network -> Offline data" list, unless the site also uses the offline cache. If the site does appear in that list, its DOM storage data is removed along with the offline cache when clicking the Remove button.

See also clearing offline resources cache.

More information

• Web Storage (W3C Web Apps Working Group)
• Enable/Disable DOM Storage in Firefox or SeaMonkey

Examples

• JavaScript Web Storage Tutorial: Creating an Address Book Application - Hands-on tutorial describing how to use the Web Storage API by creating a simple address book application.
• offline web applications at hacks.mozilla.org - Showcases an offline app demo and explains how it works.
• Noteboard - Note writing application that stores all data locally.
• jData - A shared localStorage object interface that can be accessed by any website on the internet and works on Firefox 3+, Webkit 3.1.2+ nightlies, and IE8. Think of it as pseudo-globalStorage[""] but write access needs user confirmation.
• HTML5 localStorage example - Very simple and easy to understand example of localStorage. Saves and retrieves texts and shows a list of saved items. Tested in Firefox 3 or higher.
• HTML5 Session Storage - A very simple example of session storage. Also includes a example on local storage. Tested in Firefox 3.6 or higher.
• Basic DOMStorage Examples - Broken in Firefox 3 and up due to use of globalStorage on one domain level up from the current domain which is not allowed in Firefox 3.
• halfnote - (displaying broken in Firefox 3) Note writing application that uses DOM Storage.

Browser compatibility

All browsers have varying capacity levels for both localStorage and sessionStorage. Here is a detailed rundown of all the storage capacities for various browsers.

Related

• HTTP cookies (document.cookie)
• Flash Local Storage
• Internet Explorer userData behavior
• nsIDOMStorageEventObsolete
• StorageEvent
• Implementation Details