35. Why do you need a Cookie?

In JavaScript, cookies are small pieces of data stored on the user's browser. They are primarily used for storing information that needs to persist across sessions or page reloads, such as:

1. Session Management – To maintain user login sessions.
2. Personalization – To store user preferences like themes or language settings.
3. Tracking and Analytics – To track user behavior across websites for improving user experience or targeted advertising.

Example 1: Setting a Cookie

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

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

• username=JohnDoe → Sets the key-value pair for the cookie.
• expires=Fri, 31 Dec 2025 23:59:59 GMT → Sets the expiration date.
• path=/ → Makes the cookie accessible across the entire site.

Example 2: Getting a Cookie

You can read a cookie like this:

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console.log(document.cookie);

Output:

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"username=JohnDoe"

Example 3: Deleting a Cookie

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

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

Why Use Cookies:

• Persistent storage (data remains even after the browser is closed).
• Simple API for storing and reading data.
• Useful for authentication and session handling.

Limitations:

• Cookies have size limits (~4KB per cookie).
• Cookies are sent with every HTTP request, which may affect performance.
• Vulnerable to security issues like cross-site scripting (XSS) if not handled properly.