What are HTTP cookies

A HTTP cookie is a small piece of data that a server sends to a user's web browser. The browser may then store the cookie and send it back to the same server with later requests.

Cookies are used typically to tell if two HTTP requests come from the same browser/user. So for session management, tracking and to a lesser extent personalization

For general data storage on the client, there are more modern APIs.

How cookies are sent between server and client

When client(browser) and the server communicate with HTTP calls (GET, POST etc) the calls include content, or a body and headers

The headers contain a lot of information on the request or response currently being transported. Things like content type, cache control and user agent. Headers are present both when client mades a request to the server, and when the server responds.

Cookies are also set and transported using HTTP headers. Setting cookies on the server is done by adding a Set-Cookie header to a response. The browser will then attach a Cookie header, with the value sent by the server in the Set-Cookie header, to every request it will make to a url with the same domain.

Set-Cookie has a number of optional attributes which can be used to configure how the the browser treats the cookie. These include for example Max-Age, which can be used to set the lifetime of the cookie.

Using cookies in Node.js and Express

Setting cookies in Express.js is done by calling the cookie method on a Response object.

app.get('/', (req, res) => {
    const value = "something";
    res.cookie("somecookie", value, { maxAge: 1000 * 60 * 15 })
    res.send('Hello World!')
});

Reading cookies can be done with the cookies property of the Request object When using the cookie-parser middleware, cookies becomes a object that contains the cookies senty by the browser.

var app = express()
app.use(cookieParser())
app.get('/', (req, res) => {
    const value = req.cookies.somecookie;
    res.send('Hello World!');
});

Cookie security

Cookies are automatically sent by the browser when a request is made to the domain which set the cookie originally. This is really nice, but can result in security issues. You don't want the browser to allow accessing the cookie from JavaScript or sending the cookies over an insecure connection.

To make your cookies secure, you should use the optional attributes passed to Set-Cookie.

HttpOnly
Forbids JavaScript access to cookie using Document.cookie

Secure
Sends the cookie only over HTTPS

SameSite=Strict | Lax | None
Strict: Only send the cookie for same-site requests
Lax: Don't send the cookie on cross-site requests but send it when navigating to our server using a link
None: Always sends the cookie

Expires=date
Sets the maximum lifetime of the cookie as a HTTP-date timestamp

It's important to set HttpOnly for any cookies containing authentication tokens or sensitive information to prevent JavaScript access to them in the client.

SameSite can be used to prevent some Cross-Site Request Forgery attacks. Setting it to Strict might not be ideal, because then a user navigating from a link to your site, will have to login even if they have a valid session. Setting it to Lax will only send the cookies when a user is navigating to your site SameSite defaults to Lax if not set.

If Expires is not set, the cookie will be deleted when the browser session ends.

With Express.js you can set these using the options object passed to cookie method of the Response object.

app.get('/', (req, res) => {
    const value = "something";
    res.cookie("somecookie", value, { 
        httpOnly: true,
        secure: true,
        sameSite: "strict",
        expires: date
    })
    res.send('Hello World!')
});

If you're using cookies to create a user session, consider using a middleware like express-session

References

MDN on Cookies