What is the Browser Local Storage? (Web API)
About
localStorage 1) is a browser/client side data storage mechanism.
It's one of the two web storage (key/pair) api and is part of the web api
The localStorage property allows you to access a local Storage object and therefore it implements all the functions of the Storage API (interface).
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 )
Characteristic / Security / Access
- Data stored in a localStorage object has no expiration time.
- The data is secured by origins
- Not available from pages loaded from other origins
- available to all scripts from pages that are loaded from the same origin
data stored in sessionStorage gets cleared when the browsing session ends—that is, when the browser is closed.
localStorage is protected by the same-origin policy. Any code that is not from the same origin cannot access the same data.
Use cases
- data that needs to be accessed:
- across windows or tabs,
- across multiple sessions,
- to store large (multi-megabyte) volumes of data for performance reasons
Management
Show
The devtool can show you the local storage by origin
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) );
});
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));
}