Browser - Web API - Local Storage

About

Window/localStorage of the web api is a browser/client side data storage mechanism.

The localStorage property allows you to access a local Storage object and therefore it implements all the function of the Storage API (interface).

_

Charactertistic

  • data stored in a localStorage object has no expiration time
  • you cannot list all keys, you need therefore to know the key
  • Pages on other domains can't access the same storage objects.

data stored in sessionStorage gets cleared when the browsing session ends—that is, when the browser is closed.

Example

  • Listen to storage event on other page whenever a change is made to the Storage object. It won't work on this page
window.addEventListener('storage', function(e) {  
  console.log( e.key );
  console.log( e.oldValue );
  console.log( e.newValue );
  console.log( e.url );
  console.log( JSON.stringify(e.storageArea) );
});
  • Set/Get
let key = 'key';

// value is null if the key is not found
if(!localStorage.getItem(key)) {
  localStorage.setItem(key, 'Default');
} 

// value may be null if the key is not found
let value = localStorage.getItem(key);
console.log('The value of the key is '+value )

Management

Show

The devtool can show you the local storage.

_

Length

console.log("There is "+localStorage.length+" key pairs in your local storage");

Set

localStorage.setItem(key, value);
// or
localStorage.key = value;
// or
localStorage[key] = value;

IsSet

  • value is null if the key is not found
if(!localStorage.getItem(key)) {
  // Not set
  localStorage.setItem(key, 'Default');
} 

Get

  • value may be null if the key is not found
  • You get always a string. test the string in predicate then otherwise you got a boolean problem. !'false' is false and not true.
let keyValue = localStorage.getItem(key);
// or
let keyValue = localStorage.key;
// or 
let keyValue = localStorage[key];
// or where i is the position (localStorage.length gives you the length of a localStorage)
let keyValue = localStorage.key(i);

List

console.log("There is "+localStorage.length+" key/pair that are:")
for(var i =0; i < localStorage.length; i++){
  var key = localStorage.key(i);
  var item = localStorage.getItem(key)
  console.log("  - "+(i+1)+" : "+key+" : "+JSON.stringify(item).substr(0,30));
}

Remove

  • one
localStorage.removeItem(key);
  • all
localStorage.clear();

Listen

Storage event is a way for other pages on the domain using the storage to sync any changes that are made.

This won't work on the same page that is making the changes

  • Listen to to storage event on other page whenever a change is made to the Storage object
window.addEventListener('storage', function(e) {  
  console.log( e.key );
  console.log( e.oldValue );
  console.log( e.newValue );
  console.log( e.url );
  console.log( JSON.stringify(e.storageArea) );
});

Security

localStorage is protected by the same-origin policy. Any code that is not from the same origin cannot access the same data.

Support

localStorage is not defined

You may get this error when javascript is running on Node and not in the browser.

To make your script, Node proof, add this following condition:

if (typeof window != 'undefined') {
  // whatever with localstorage
  window.localStorage.setItem(key, JSON.stringify(state));
}

Documentation


Powered by ComboStrap