Web Storage is one of the great features of HTML5. With the Web Storage feature, web applications can locally store data within the browser on the client-side. It stores data in the form of key/value pair on the browser. Web Storage is sometimes also known as DOM storage.

With web storage, web applications can store data locally within the user’s browser.

Before HTML5, application data had to be stored in cookies, included in every server request. Web storage is more secure, and large amounts of data can be stored locally, without affecting website performance.

Unlike cookies, the storage limit is far larger (at least 5MB) and information is never transferred to the server.

Web storage is per origin (per domain and protocol). All pages, from one origin, can store and access the same data.

Types of Web Storage

There are two types of web storage with different scopes and lifetimes.

  1. Local Storage: Local Storages uses Windows.localStaorage object which stores data and available for every page. But data persist even if the browser is closed and reopened (Stores data with no Expiration). 
  2. Session Storage: Session Storage uses Windows.sessionStorage object which stores data for one session and data will be lost if the window or browser tab will be closed.

Browser Support

The numbers in the table specify the first browser version that fully supports Web Storage.

HTML Web Storage Objects

HTML web storage provides two objects for storing data on the client:

  1. window.localStorage – stores data with no expiration date
  2. window.sessionStorage – stores data for one session (data is lost when the browser tab is closed)

Before using web storage, check browser support for localStorage and sessionStorage:

if (typeof(Storage) !== "undefined") {
  // Code for localStorage/sessionStorage.
} else {
  // Sorry! No Web Storage support..
}

The localStorage Object

The localStorage object stores the data with no expiration date. The data will not be deleted when the browser is closed and will be available the next day, week, or year.

Example

// Store
localStorage.setItem("lastname", "Smith");

// Retrieve
document.getElementById("result").innerHTML = localStorage.getItem("lastname");

Example explained:

  1. Create a localStorage name/value pair with name=”lastname” and value=”Smith”
  2. Retrieve the value of “lastname” and insert it into the element with id=”result”

The example above could also be written like this:

// Store
localStorage.lastname = "Smith";
// Retrieve
document.getElementById("result").innerHTML = localStorage.lastname;

The syntax for removing the “lastname” localStorage item is as follows:

localStorage.removeItem("lastname");

Note: Name/value pairs are always stored as strings. Remember to convert them to another format when needed!

The following example counts the number of times a user has clicked a button. In this code the value string is converted to a number to be able to increase the counter:

Example

if (localStorage.clickcount) {
  localStorage.clickcount = Number(localStorage.clickcount) + 1;
} else {
  localStorage.clickcount = 1;
}
document.getElementById("result").innerHTML = "You have clicked the button " +
localStorage.clickcount + " time(s).";

The sessionStorage Object

The sessionStorage object is equal to the localStorage object, except that it stores the data for only one session. The data is deleted when the user closes the specific browser tab.

The following example counts the number of times a user has clicked a button, in the current session:

Example

if (sessionStorage.clickcount) {
  sessionStorage.clickcount = Number(sessionStorage.clickcount) + 1;
} else {
  sessionStorage.clickcount = 1;
}
document.getElementById("result").innerHTML = "You have clicked the button " +
sessionStorage.clickcount + " time(s) in this session.";

Private Browsing / Incognito modes

Most modern browsers support a privacy option called ‘Incognito’, ‘Private Browsing’, or something similar that doesn’t store data like history and cookies. This is fundamentally incompatible with Web Storage for obvious reasons. As such, browser vendors are experimenting with different scenarios for how to deal with this incompatibility.

Most browsers have opted for a strategy where storage APIs are still available and seemingly fully functional, with the one big difference being that all stored data is wiped after the browser is closed. For these browsers, there are still different interpretations of what should be done with existing stored data (from a regular browsing session). Should it be available to read when in Private mode? Then there are some browsers, most notably Safari, that have opted for a solution where storage is available, but is empty and has a quota of 0 bytes assigned, effectively making it impossible to write data to it.

Developers should be aware of these different implementations and take them into account when developing websites depending on Web Storage APIs.