HTML5 Web Storage

if (Modernizr.localstorage) { //use localStorage object to store data } else { alert('Cannot store user preferences as your browser do not support local storage'); } The setItem(‘key’,’value’) allows us to write the data into the local storage. QUOTA_EXCEEDED_ERR exception will be thrown if the storage limit exceeds 5MB. So it is always better to add try/catch blocks to the storage code while saving data.

<span>function setSettings() {
</span><span>if ('localStorage' in window && window['localStorage'] !== null) {
</span>	<span>try {
</span>		<span>var favcolor = document.getElementById('favcolor').value;
</span>		<span>var fontwt = document.getElementById('fontwt').value;
</span>		<span>localStorage.setItem('bgcolor', favcolor);
</span>		<span>localStorage.fontweight = fontwt;
</span>	<span>} catch (e) {
</span>		<span>if (e == QUOTA_EXCEEDED_ERR) {
</span>			<span>alert('Quota exceeded!');
</span>		<span>}
</span>	<span>}
</span>	<span>} else {
</span>		<span>alert('Cannot store user preferences as your browser do not support local storage');
</span>	<span>}
</span><span>}</span>

We can check whether the data is stored in local storage by using the developer tools that comes with the browsers. For instance, in Chrome, right click on the browser and select Inspect Element. Select Resources tab and then click on the local storage item. We can see the user selected data stored in the form of key/value pairs. The web page can be reloaded with the background and font size values set by the user. The getItem(‘Key’) helps in retrieving the data stored In the database.

<span>function applySetting() {
</span>	<span>if (localStorage.length != 0) {
</span>	<span>document.body.style.backgroundColor = localStorage.getItem('bgcolor');
</span>	<span>document.body.style.fontSize = localStorage.fontweight + 'px';
</span>	<span>document.getElementById('favcolor').value = localStorage.bgcolor;
</span>	<span>document.getElementById('fontwt').value = localStorage.fontweight;
</span>	<span>} else {
</span>	<span>document.body.style.backgroundColor = '#FFFFFF';
</span>	<span>document.body.style.fontSize = '13px'
</span>	<span>document.getElementById('favcolor').value = '#FFFFFF';
</span>	<span>document.getElementById('fontwt').value = '13';
</span>	<span>}
</span><span>}</span>

The length function retrieves the total number of values in the storage area. The above function can be called during the onload event of the body tag as follows The local storage area can be cleared by using the clear() function or removeItem(‘key’) function. In our example the function below is called on the click event of the clear button.

<span>function clearSettings() {
</span>		<span>localStorage.removeItem("bgcolor");
</span>		<span>localStorage.removeItem("fontweight");
</span>		<span>document.body.style.backgroundColor = '#FFFFFF';
</span>		<span>document.body.style.fontSize = '13px'
</span>		<span>document.getElementById('favcolor').value = '#FFFFFF';
</span>		<span>document.getElementById('fontwt').value = '13';
</span>
<span>}</span>

Storage Events

When we set or remove data from the web storage, a storage event will be fired on the window object. We can add listeners to the event and handle the storage changes if required.

<span>window.addEventListener('storage', storageEventHandler, false);
</span>	<span>function storageEventHandler(event) {
</span>			<span>applySetting();
</span>	<span>}</span>

The event object has the following attributes

• key – the property that has changed
• newValue – the newly set value
• oldValue –  previously stored  value
• url – the full url path from where the event originated
• storageArea –  localStorage or sessionStorage object

Remember that the event gets fired only on other windows (not on the window where the event is triggered) and the event does not get fired if there is no change in the data. The same API methods are applicable for session storage as well except that the methods should be executed on the sessionStorage object.

Conclusion

So, now you can start using Web Storage to store user preferences, user info, session info etc.You can also try creating apps that can be used completely offline and the data stored during offline can be sent back to the server as a batch update when the user is online again.

Frequently Asked Questions (FAQs) about HTML5 Web Storage

What is the difference between sessionStorage and localStorage in HTML5 Web Storage?

In HTML5 Web Storage, there are two types of storage: sessionStorage and localStorage. The main difference between them lies in their lifespan and scope. sessionStorage is designed to be a temporary storage for the duration of a single browser session. It gets cleared as soon as the session ends, i.e., when the user closes the browser tab or window. On the other hand, localStorage persists even when the browser is closed and reopened. It has no expiration time and remains until it is manually cleared by the user or the web application.

How can I access and manipulate data in HTML5 Web Storage?

Accessing and manipulating data in HTML5 Web Storage is straightforward. You can use the setItem() method to store data, getItem() method to retrieve data, and removeItem() method to delete data. For example, to store a data item in localStorage, you can use localStorage.setItem(‘key’, ‘value’). To retrieve this data, use localStorage.getItem(‘key’). To delete the data, use localStorage.removeItem(‘key’).

Is HTML5 Web Storage secure?

HTML5 Web Storage is secure to an extent. It does not allow storage of sensitive user information like passwords or credit card numbers. However, it is susceptible to cross-site scripting (XSS) attacks. Therefore, it’s recommended to not store sensitive information and always validate and sanitize your data before storing it.

What is the storage limit of HTML5 Web Storage?

The storage limit for HTML5 Web Storage varies between different browsers. However, most modern browsers offer around 5MB of storage per domain for localStorage. sessionStorage also offers the same amount of storage but it’s important to remember that this storage is temporary.

Can I use HTML5 Web Storage on all browsers?

HTML5 Web Storage is supported by all modern browsers including Chrome, Firefox, Safari, Opera, and Internet Explorer 8 and above. However, it’s always a good practice to check for browser compatibility before using it.

How can I check if a browser supports HTML5 Web Storage?

You can check if a browser supports HTML5 Web Storage by using a simple ‘if’ condition in your JavaScript code. If(typeof(Storage) !== “undefined”) { // Code for localStorage/sessionStorage. } else { // Sorry! No Web Storage support.. }

Can I store objects or arrays in HTML5 Web Storage?

Yes, you can store objects or arrays in HTML5 Web Storage. However, since Web Storage only supports string values, you need to convert your objects or arrays into strings using JSON.stringify() before storing them. To retrieve them, you can convert them back into objects or arrays using JSON.parse().

How can I clear all data in HTML5 Web Storage?

You can clear all data in HTML5 Web Storage using the clear() method. For example, to clear all data in localStorage, you can use localStorage.clear().

Can I use HTML5 Web Storage for offline applications?

Yes, HTML5 Web Storage can be used for offline applications. It allows you to store data on the user’s browser, which can then be accessed and used even when the user is offline.

What are the alternatives to HTML5 Web Storage?

There are several alternatives to HTML5 Web Storage, including cookies, IndexedDB, and Web SQL. However, each of these has its own advantages and disadvantages, and the choice depends on the specific requirements of your web application.

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