JavaScript Cookies

JavaScript Cookies

In JavaScript Cookies is a small piece of data stored in the browser.
Cookies are often used to:

Save user preferences
Track sessions
Store login information

Unlike LocalStorage/SessionStorage, cookies are sent to the server with every HTTP request.

Create a Cookie

This creates a cookie named username with value Vipul.

Set Cookie with Expiration

expires – Sets expiration date
path=/ – Makes cookie accessible on the entire website

Read Cookies

Get a specific cookie

Delete a Cookie

To delete a cookie, set its expiration date to a past date:

Secure and HttpOnly Cookies

Secure – Only sent over HTTPS
HttpOnly – Not accessible via JavaScript (for security)

Example:

Note: HttpOnly cookies cannot be created via JavaScript. They must be set by the server.

Example: Cookie Storage for User Theme

<button onclick="setDarkMode()">Dark Mode</button>
<button onclick="setLightMode()">Light Mode</button>

<script>
function setDarkMode() {
document.cookie = "theme=dark; path=/";
document.body.style.backgroundColor = "black";
}

function setLightMode() {
document.cookie = "theme=light; path=/";
document.body.style.backgroundColor = "white";
}

// Apply theme on page load
let theme = getCookie("theme");
if(theme === "dark") document.body.style.backgroundColor = "black";

function getCookie(name) {
let cookies = document.cookie.split("; ");
for(let cookie of cookies) {
let [key, value] = cookie.split("=");
if(key === name) return value;
}
return "";
}
</script>

<button onclick="setLightMode()">Light Mode</button>

function setDarkMode() {

document.cookie = "theme=dark; path=/";

document.body.style.backgroundColor = "black";

}

function setLightMode() {

document.cookie = "theme=light; path=/";

document.body.style.backgroundColor = "white";

}

// Apply theme on page load

let theme = getCookie("theme");

if(theme === "dark") document.body.style.backgroundColor = "black";

function getCookie(name) {

let cookies = document.cookie.split("; ");

for(let cookie of cookies) {

let [key, value] = cookie.split("=");

if(key === name) return value;

}

return "";

}

</script>

Cookies vs Web Storage

Feature	Cookies	LocalStorage	SessionStorage
Storage Size	~4KB	~5MB	~5MB
Expiration	Can set expiry	Until manually cleared	Until tab closed
Accessible by server?	Yes	No	No
Used for	Authentication, small data	Large client-side data	Temporary session data

Summary

Use cookies for server communication (sessions, login)
Use LocalStorage/SessionStorage for larger client-side data
Always manage expiration dates
Avoid storing sensitive data in client-accessible cookies

JavaScript Cookies