34. What is a Cookie?

In JavaScript, a cookie is a small piece of data stored in the user's browser. Cookies are used to store information such as user preferences, session tokens, and tracking data between browser sessions.

Setting a Cookie:

You can set a cookie using the document.cookie property:

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document.cookie = "username=John; expires=Fri, 31 Dec 2025 23:59:59 GMT; path=/";

Explanation:

• username=John – Sets the name-value pair for the cookie.
• expires=... – Sets the expiration date for the cookie (optional). If not set, the cookie is deleted when the browser is closed.
• path=/ – Makes the cookie accessible for all paths on the domain (optional).

Reading a Cookie:

You can read cookies using document.cookie:

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console.log(document.cookie);

Output:

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username=John

Deleting a Cookie:

To delete a cookie, set the expires date to a past date:

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

Example:

Here's a complete example:

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// Set a cookie
document.cookie = "theme=dark; expires=Fri, 31 Dec 2025 23:59:59 GMT; path=/";

// Read a cookie
console.log(document.cookie); // Output: theme=dark

// Delete a cookie
document.cookie = "theme=dark; expires=Thu, 01 Jan 1970 00:00:00 UTC; path=/";
console.log(document.cookie); // Output: (empty)

Note:

• Cookies are limited to about 4KB of data.
• Secure and HttpOnly flags can enhance security but are set on the server side.