JavaScript Cookies

What are cookies?

Cookies are data,stored in small text files,on your computer.

When a web server has sent a web page to a browser, the connection is shut down, and the server forgets everything about the user.

Cookies were invented to solve the problem "how to remember information about the user":

When a user visits a web page, his/her name can be stored in
a cookie.
Next time the user visits the page, the cookie "remembers"
his/her name.
Cookies are saved in name-value pairs like:

username = John Doe

When a browser requests a web page from a server, cookies belonging to the page are added to the request. This way the server gets the necessary data to "remember" information about users.

NOTE: None of the examples below will work if your browser has local cookies support turned off.

Create a Cookie with JavaScript:
JavaScript can create, read, and delete cookies with the document.cookie property.

With JavaScript, a cookie can be created like this:

document.cookie = "username=John Doe";

You can also add an expiry date (in UTC time). By default, the cookie is deleted when the browser is closed:

document.cookie = "username=John Doe; expires=Thu, 18 Dec 2013 12:00:00 UTC";

With a path parameter, you can tell the browser what path the cookie belongs to. By default, the cookie belongs to the current page.

document.cookie = "username=John Doe; expires=Thu, 18 Dec 2013 12:00:00 UTC; path=/";

Read a Cookie with JavaScript:

With JavaScript, cookies can be read like this:

let x = document.cookie;

document.cookie will return all cookies in one string much like: cookie1=value; cookie2=value; cookie3=value;

Change a Cookie with JavaScript:
With JavaScript, you can change a cookie the same way as you create it:

document.cookie = "username=John Smith; expires=Thu, 18 Dec 2013 12:00:00 UTC; path=/";

The old cookie is overwritten.

Delete a Cookie with JavaScript:

Deleting a cookie is very simple.

You don't have to specify a cookie value when you delete a cookie.

Just set the expires parameter to a past date:

document.cookie = "username=; expires=Thu, 01 Jan 1970 00:00:00 UTC; path=/;";

NOTE:You should define the cookie path to ensure that you delete the right cookie.
Some browsers will not let you delete a cookie if you don't specify the path.

The Cookie String:

The document.cookie property looks like a normal text string. But it is not.

Even if you write a whole cookie string to document.cookie, when you read it out again, you can only see the name-value pair of it.

If you set a new cookie, older cookies are not overwritten. The new cookie is added to document.cookie, so if you read document.cookie again you will get something like:

cookie1 = value; cookie2 = value;

JavaScript Cookie Example:

In the example to follow, we will create a cookie that stores the name of a visitor.

The first time a visitor arrives to the web page, he/she will be asked to fill in his/her name. The name is then stored in a cookie.

The next time the visitor arrives at the same page, he/she will get a welcome message.

For the example we will create 3 JavaScript functions:

A function to set a cookie valueA function to get a cookie valueA function to check a cookie value

A function to set a cookie:

function setCookie(cname, cvalue, exdays) {
const d = new Date();
d.setTime(d.getTime() + (exdays*24*60*60*1000));
let expires = "expires="+ d.toUTCString();
document.cookie = cname + "=" + cvalue + ";" + expires + ";path=/";
}

A function to get a cookie:

function getCookie(cname) {
let name = cname + "=";
let decodedCookie = decodeURIComponent(document.cookie);
let ca = decodedCookie.split(';');
for(let i = 0; i <ca.length; i++) {
let c = ca[i];
while (c.charAt(0) == ' ') {
c = c.substring(1);
}
if (c.indexOf(name) == 0) {
return c.substring(name.length, c.length);
}
}
return "";
}

A Function to Check a Cookie:

function checkCookie() {
let username = getCookie("username");
if (username != "") {
alert("Welcome again " + username);
} else {
username = prompt("Please enter your name:", "");
if (username != "" && username != null) {
setCookie("username", username, 365);
}
}
}