HTML5 adds a new feature to browsers which allows them to store a certain amount of data locally on someone's computer. This comes in handy with some of the recent "web apps" that might, for example, save your drafts or states even when disconnecting from the internet. This feature is a Javascript API known as localStorage, and in this tutorial, we'll be covering some of the basics of how it works.

The API

Let's clear up some of the ideas around localStorage. You might be aware of "cookies", which sites use to remember you or your login. Cookies are very limited in size, and really aren't meant to store actual data. localStorage, on the other hand, is meant for sites to have a 5-10MB space on your PC to save whatever they may need. This storage is NOT sent to the server, in contrast to cookies. This means all data stored with localStorage is kept on that one computer, and only accessible by the browser you used to create the data. Every modern browser supports this, so let's look into the code (Javascript knowledge is needed):

You can access localStorage with window.localStorage. If you type that into a console, you'll probably see that the current site you call it from isn't using it.

Firebug console

The way localStorage actually stores its data is with string key/value pairs. You either set or get the values by their key string. For example, I might add a key named "user", and set its value to "John": window.localStorage.setItem('user','John'). Now if you call window.localStorage you'll see you have 1 item.

setItem with localStorage

Now that you've set an item, you can also get it's value by calling it by its string: window.localStorage.getItem('user')

getItem with localStorage

One more method to add to these localStorage basics would be window.localStorage.clear(), which as you might guess, clears out everything in localStorage.

Clear localStorage

Simple Site

So far everything seems pretty simple, right? Let's see a simple, real world example of how we can use this. Let's make a small notepad type app, that will store everything we type into it, into our localStorage. Let's set up some quick markup first:






localStorage Notepad




Which would give us a page like so:

Notepad page

You might notice at the end I decided to use Zepto.js from a CDN, basically I'm just using it to slim down on the Javascript we have to write. Anyway, you'll notice that at the moment all we have is a div that you can type into, but that's about it. Once you refresh the page, the notepad is blank once again. Let's plug in some localStorage magic!

Store it!

You'll see we've added some extra stuff to our Javascript. Zepto acts just like jQuery for the most part, so upon initialization we first set an alias to window.localStorage, that way we can just use ls instead of window.localStorage typing out over and over. The next thing we do is a bind a function to the blur event (i.e. when you remove focus) of the .notepad div. In this case, we want the page to save the text that it contains whenever we remove focus from the pad. Now you can call window.localStorage after you type new text and see that it's being saved locally!

Notepad setItem to text

Read it too

Great, we've added saving to our notepad app, but if you refresh the notepad is still blank like before. However, if you call window.localStorage in a console, you will see that the saved text still exists. All we need to do is add a little more JS to retrieve any saved data and apply it on load!

With that middle section, you'll see we first check for the existence of any saved text (excluding line breaks) under the "data" key, and if so, apply it to the notepad div. With that, you now have a super simple notepad app using some localStorage basics!