The localStorage object provides persistent (but not permanent - see limits below) key-value storage of strings. Any changes are immediately visible in all other windows/frames from the same origin. The stored values persistent indefinitely unless the user clears saved data or configures an expiration limit. localStorage uses a map-like interface for getting and setting values.

localStorage.setItem('name', "John Smith");
console.log(localStorage.getItem('name')); // "John Smith"

localStorage.removeItem('name');
console.log(localStorage.getItem('name')); // null

If you want to store simple structured data, you can use JSON to serialize it to and from strings for storage.

var players = [{name: "Tyler", score: 22}, {name: "Ryan", score: 41}];
localStorage.setItem('players', JSON.stringify(players));

console.log(JSON.parse(localStorage.getItem('players')));
// [ Object { name: "Tyler", score: 22 }, Object { name: "Ryan", score: 41 } ]

localStorage limits in browsers

Mobile browsers:

Browser	Google Chrome	Android Browser	Firefox	iOS Safari
Version	40	4.3	34	6-8
Space available	10MB	2MB	10MB	5MB

Desktop browsers:

Browser	Google Chrome	Opera	Firefox	Safari	Internet Explorer
Version	40	27	34	6-8	9-11
Space available	10MB	10MB	10MB	5MB	10MB

Whenever a value in set in localStorage, a storage event will be dispatched on all other windows from the same origin. This can be used to synchronize state between different pages without reloading or communicating with a server. For example, we can reflect the value of an input element as paragraph text in another window:

First Window

var input = document.createElement('input');
document.body.appendChild(input);

input.value = localStorage.getItem('user-value');

input.oninput = function(event) {
  localStorage.setItem('user-value', input.value);
};

Second Window

var output = document.createElement('p');
document.body.appendChild(output);

output.textContent = localStorage.getItem('user-value');

window.addEventListener('storage', function(event) {
  if (event.key === 'user-value') {
    output.textContent = event.newValue;
  }
});

Notes

Event is not fired or catchable under Chrome, Edge and Safari if domain was modified through script.

First window

// page url: http://sub.a.com/1.html
document.domain = 'a.com';

var input = document.createElement('input');
document.body.appendChild(input);

input.value = localStorage.getItem('user-value');

input.oninput = function(event) {
  localStorage.setItem('user-value', input.value);
};

Second Window

// page url: http://sub.a.com/2.html
document.domain = 'a.com';

var output = document.createElement('p');
document.body.appendChild(output);

// Listener will never called under Chrome(53), Edge and Safari(10.0).
window.addEventListener('storage', function(event) {
  if (event.key === 'user-value') {
    output.textContent = event.newValue;
  }
});

The sessionStorage object implements the same Storage interface as localStorage. However, instead of being shared with all pages from the same origin, sessionStorage data is stored separately for every window/tab. Stored data persists between pages in that window/tab for as long as it's open, but is visible nowhere else.

var audio = document.querySelector('audio');

// Maintain the volume if the user clicks a link then navigates back here.
audio.volume = Number(sessionStorage.getItem('volume') || 1.0);
audio.onvolumechange = function(event) {
  sessionStorage.setItem('volume', audio.volume);
};

Save data to sessionStorage

sessionStorage.setItem('key', 'value');

Get saved data from sessionStorage

var data = sessionStorage.getItem('key');

Remove saved data from sessionStorage

sessionStorage.removeItem('key')

To clear the storage, simply run

localStorage.clear();

Most browsers, when configured to block cookies, will also block localStorage. Attempts to use it will result in an exception. Do not forget to manage these cases.

var video = document.querySelector('video')
try {
    video.volume = localStorage.getItem('volume')
} catch (error) {
    alert('If you\'d like your volume saved, turn on cookies')
}
video.play()

If error were not handled, program would stop functioning properly.

To remove a specific item from the browser Storage (the opposite of setItem) use removeItem

localStorage.removeItem("greet");

Example:

localStorage.setItem("greet", "hi");
localStorage.removeItem("greet");

console.log( localStorage.getItem("greet") ); // null

(Same applies for sessionStorage)

localStorage, sessionStorage are JavaScript Objects and you can treat them as such.
Instead of using Storage Methods like .getItem(), .setItem(), etc… here's a simpler alternative:

// Set
localStorage.greet = "Hi!"; // Same as: window.localStorage.setItem("greet", "Hi!");

// Get
localStorage.greet;         // Same as: window.localStorage.getItem("greet");

// Remove item
delete localStorage.greet;  // Same as: window.localStorage.removeItem("greet");

// Clear storage
localStorage.clear();

Example:

// Store values (Strings, Numbers)
localStorage.hello = "Hello";
localStorage.year  = 2017;    

// Store complex data (Objects, Arrays)
var user = {name:"John", surname:"Doe", books:["A","B"]};
localStorage.user = JSON.stringify( user );

// Important: Numbers are stored as String
console.log( typeof localStorage.year ); // String

// Retrieve values
var someYear = localStorage.year; // "2017"

// Retrieve complex data
var userData = JSON.parse( localStorage.user );
var userName = userData.name; // "John"

// Remove specific data
delete localStorage.year; 

// Clear (delete) all stored data
localStorage.clear();

localStorage.length property returns an integer number indicating the number of elements in the localStorage

Example:

Set Items

localStorage.setItem('StackOverflow', 'Documentation');
localStorage.setItem('font', 'Helvetica');
localStorage.setItem('image', 'sprite.svg');

Get length

localStorage.length; // 3