LocalStorage

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localStorage 与 sessionStorage 一样,都遵循同源策略,但是它是持续存在的localStorage 首次出现于 Firefox 3.5。

Note: 当浏览器进入隐私浏览模式,会创建一个新的、临时的数据库来存储local storage的数据;当关闭隐私浏览模式时,该数据库将被清空并丢弃。
// Save data to the current local store
localStorage.setItem("username", "John");

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

本地存储(localStorage)的持续存在对许多事情都有帮助,包括这个示例this tutorial on Codepen所演示的记录页面查看次数。

兼容性

Storage 对象最近加入了标准,因此可能并不被所有浏览器支持。你可以通过在你的scripts代码前加入以下两段代码中某一段来规避在不能原生支持的执行环境使用localStorage对象的问题

该算法借助于cookies,对localStorage对象进行了严谨的实现。

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;
  })());
}
注意:数据的最大大小是通过cookies来严格限制的。可以使用这样的算法实现,使用localStorage.setItem()和localStorage.removeItem()这两个函数进行添加、改变或删除一个键。使用方法为“localStorage.yourKey = yourValue;”或者删除一个键“localStorage.yourKey;”。使用这种方法设置和删除一个键并不是安全的做法。您也可以改变它的名字和使用它仅仅去管理文档的cookies而不管localStorage 这个对象。
注意:通过改变字符串的方法,比如如将字符串"; expires=Tue, 19 Jan 2038 03:14:07 GMT; path=/" 改成"; path=/"(和改变对象的名字)this will become a sessionStorage polyfill rather than a localStorage polyfill. However, this implementation will share stored values across browser tabs and windows (and will only be cleared when all browser windows have been closed), while a fully-compliant sessionStorage implementation restricts stored values to the current browsing context only.

下面是另一个,并不那么严谨的localStorage对象的实现。它比上一个方法更简单且能与旧的浏览器良好兼容,如Internet Explorer < 8 (甚至能在 Internet Explorer 6 上正常工作)。它同样是使用cookies来实现的。

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;
}
Note: The maximum size of data that can be saved is severely restricted by the use of cookies. With this algorithm, use the functions localStorage.getItem()localStorage.setItem(), and localStorage.removeItem() to get, add, change, or remove a key. The use of method localStorage.yourKey in order to get, set, or delete a key is not permitted with this code. You can also change its name and use it only to manage a document's cookies regardless of the localStorage object.
Note: By changing the string "; expires=Tue, 19 Jan 2038 03:14:07 GMT; path=/" to: "; path=/" (and changing the object's name), this will become a sessionStorage polyfill rather than a localStorage polyfill. However, this implementation will share stored values across browser tabs and windows (and will only be cleared when all browser windows have been closed), while a fully-compliant sessionStorage implementation restricts stored values to the current browsing context only.

与globalStorage的兼容性及关系

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, http://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.

请注意,给globalStorage[location.hostname]设置一个属性并不会影响到localStorage。扩充Storage.prototype也不会影响globalStorage对象,只有扩充StorageObsolete.prototype才会影响。

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