The server sends the following in its response header to set a cookie field. If there is a cookie set, then the browser sends the following in its request header. See the HTTP Cookie article at Wikipedia for more information. Instantly turn your artwork into a cookie cutter Buy your custom cutter for $5 or 3D print it at home for free Create your cutter Start by uploading your own image. Black and white line art or drawings work best. What can I upload? Click here or drag files to upload. Cookies and website data are deleted unless you visit and interact with the trackers’ websites. Always block cookies: Select “Block all cookies.” Websites, third parties, and advertisers can’t store cookies and other data on your Mac. This may prevent some websites from working properly. Always allow cookies: Deselect “Block all. Cookie labels: Place a label next to each plate of cookies so they’re easily identifiable. List any allergies. Recipe cards: Have a handful available so that guests can write down any favorites. Or you could have your guest bring a printed copy. Voting ballots: Have everyone vote on the top cookie to win a fun prize!
Cookies are small strings of data that are stored directly in the browser. They are a part of HTTP protocol, defined by RFC 6265 specification.
Cookies are usually set by a web-server using response
Set-Cookie
HTTP-header. Then the browser automatically adds them to (almost) every request to the same domain using Cookie
HTTP-header.One of the most widespread use cases is authentication:
- Upon sign in, the server uses
Set-Cookie
HTTP-header in the response to set a cookie with a unique “session identifier”. - Next time when the request is set to the same domain, the browser sends the cookie over the net using
Cookie
HTTP-header. - So the server knows who made the request.
We can also access cookies from the browser, using
document.cookie
property.There are many tricky things about cookies and their options. In this chapter we’ll cover them in detail.
Reading from document.cookie
Does your browser store any cookies from this site? Let’s see:
The value of
document.cookie
consists of name=value
pairs, delimited by ;
. Each one is a separate cookie.If You Give A Mouse A Cookie Format
To find a particular cookie, we can split
document.cookie
by ;
, and then find the right name. We can use either a regular expression or array functions to do that.We leave it as an exercise for the reader. Also, at the end of the chapter you’ll find helper functions to manipulate cookies.
Writing to document.cookie
We can write to
document.cookie
. But it’s not a data property, it’s an accessor (getter/setter). An assignment to it is treated specially.A write operation to
document.cookie
updates only cookies mentioned in it, but doesn’t touch other cookies.For instance, this call sets a cookie with the name
user
and value John
:If you run it, then probably you’ll see multiple cookies. That’s because
document.cookie=
operation does not overwrite all cookies. It only sets the mentioned cookie user
.Technically, name and value can have any characters, to keep the valid formatting they should be escaped using a built-in
encodeURIComponent
function:There are few limitations:
- The
name=value
pair, afterencodeURIComponent
, should not exceed 4KB. So we can’t store anything huge in a cookie. - The total number of cookies per domain is limited to around 20+, the exact limit depends on a browser.
Cookies have several options, many of them are important and should be set.
The options are listed after
key=value
, delimited by ;
, like this:path
path=/mypath
The url path prefix, the cookie will be accessible for pages under that path. Must be absolute. By default, it’s the current path.
If a cookie is set with
path=/admin
, it’s visible at pages /admin
and /admin/something
, but not at /home
or /adminpage
.Usually, we should set
path
to the root: path=/
to make the cookie accessible from all website pages.domain
domain=site.com
A domain where the cookie is accessible. In practice though, there are limitations. We can’t set any domain.
By default, a cookie is accessible only at the domain that set it. So, if the cookie was set by
site.com
, we won’t get it other.com
.…But what’s more tricky, we also won’t get the cookie at a subdomain
forum.site.com
!There’s no way to let a cookie be accessible from another 2nd-level domain, so
other.com
will never receive a cookie set at site.com
.It’s a safety restriction, to allow us to store sensitive data in cookies, that should be available only on one site.
…But if we’d like to allow subdomains like
forum.site.com
get a cookie, that’s possible. When setting a cookie at site.com
, we should explicitly set domain
option to the root domain: domain=site.com
:For historical reasons,
domain=.site.com
(a dot before site.com
) also works the same way, allowing access to the cookie from subdomains. That’s an old notation, should be used if we need to support very old browsers.So,
domain
option allows to make a cookie accessible at subdomains.expires, max-age
By default, if a cookie doesn’t have one of these options, it disappears when the browser is closed. Such cookies are called “session cookies”
To let cookies survive browser close, we can set either
expires
or max-age
option.expires=Tue, 19 Jan 2038 03:14:07 GMT
Cookie expiration date, when the browser will delete it automatically.
The date must be exactly in this format, in GMT timezone. We can use
date.toUTCString
to get it. For instance, we can set the cookie to expire in 1 day:If we set
expires
to a date in the past, the cookie is deleted.max-age=3600
![Cookie Format Cookie Format](https://user-images.githubusercontent.com/7819909/34016478-06f99dde-e0f0-11e7-97c3-8eebc945c8cf.png)
An alternative to
expires
, specifies the cookie expiration in seconds from the current moment.If zero or negative, then the cookie is deleted:
secure
secure
The cookie should be transferred only over HTTPS.
By default, if we set a cookie at
http://site.com
, then it also appears at https://site.com
and vice versa.That is, cookies are domain-based, they do not distinguish between the protocols.
With this option, if a cookie is set by
https://site.com
, then it doesn’t appear when the same site is accessed by HTTP, as http://site.com
. So if a cookie has sensitive content that should never be sent over unencrypted HTTP, then the flag is the right thing.samesite
That’s another security attribute
samesite
. It’s designed to protect from so-called XSRF (cross-site request forgery) attacks.To understand how it works and when it’s useful, let’s take a look at XSRF attacks.
XSRF attack
Imagine, you are logged into the site
bank.com
. That is: you have an authentication cookie from that site. Smultron 10 1 7 download free. Your browser sends it to bank.com
with every request, so that it recognizes you and performs all sensitive financial operations.Now, while browsing the web in another window, you accidentally come to another site
evil.com
. That site has JavaScript code that submits a form <form action='https://bank.com/pay'>
to bank.com
with fields that initiate a transaction to the hacker’s account.The browser sends cookies every time you visit the site
bank.com
, even if the form was submitted from evil.com
. So the bank recognizes you and actually performs the payment.That’s called a “Cross-Site Request Forgery” (in short, XSRF) attack.
Real banks are protected from it of course. All forms generated by
bank.com
have a special field, so called “XSRF protection token”, that an evil page can’t generate or extract from a remote page (it can submit a form there, but can’t get the data back). And the site bank.com
checks for such token in every form it receives.But such protection takes time to implement: we need to ensure that every form has the token field, and we must also check all requests.
Enter cookie samesite option
The cookie
samesite
option provides another way to protect from such attacks, that (in theory) should not require “xsrf protection tokens”.It has two possible values:
samesite=strict
(same assamesite
without value)
A cookie with
samesite=strict
is never sent if the user comes from outside the same site.In other words, whether a user follows a link from their mail or submits a form from
evil.com
, or does any operation that originates from another domain, the cookie is not sent.If authentication cookies have
samesite
option, then XSRF attack has no chances to succeed, because a submission from evil.com
comes without cookies. Wondershare recoverit video repair 1 0 0 20 download free. So bank.com
will not recognize the user and will not proceed with the payment.The protection is quite reliable. Only operations that come from
bank.com
will send the samesite
cookie, e.g. a form submission from another page at bank.com
.Although, there’s a small inconvenience.
When a user follows a legitimate link to
bank.com
, like from their own notes, they’ll be surprised that bank.com
does not recognize them. Indeed, samesite=strict
cookies are not sent in that case.We could work around that by using two cookies: one for “general recognition”, only for the purposes of saying: “Hello, John”, and the other one for>.
Along with the banner, the remote server at
ads.com
may set Set-Cookie
header with cookie like id=1234
. Such cookie originates from ads.com
domain, and will only be visible at ads.com
:Next time when
ads.com
is accessed, the remote server gets the id
cookie and recognizes the user:What’s even more important, when the users moves from
site.com
to another site other.com
that also has a banner, then ads.com
gets the cookie, as it belongs to ads.com
, thus recognizing the visitor and tracking him as he moves between sites:Third-party cookies are traditionally used for tracking and ads services, due to their nature. They are bound to the originating domain, so
ads.com
can track the same user between different sites, if they all access it.Naturally, some people don’t like being tracked, so browsers allow to disable such cookies.
Also, some modern browsers employ special policies for such cookies:
- Safari does not allow third-party cookies at all.
- Firefox comes with a “black list” of third-party domains where it blocks third-party cookies.
If we load a script from a third-party domain, like
<script src='https://google-analytics.com/analytics.js'>
, and that script uses document.cookie
to set a cookie, then such cookie is not third-party.If a script sets a cookie, then no matter where the script came from – the cookie belongs to the domain of the current webpage.
Appendix: GDPR
This topic is not related to JavaScript at all, just something to keep in mind when setting cookies.
There’s a legislation in Europe called GDPR, that enforces a set of rules for websites to respect users’ privacy. And one of such rules is to require an explicit permission for tracking cookies from a user.
Please note, that’s only about tracking/identifying/authorizing cookies.
So, if we set a cookie that just saves some information, but neither tracks nor identifies the user, then we are free to do it.
But if we are going to set a cookie with an authentication session or a tracking id, then a user must allow that.
Websites generally have two variants of following GDPR. You must have seen them both already in the web:
- If a website wants to set tracking cookies only for authenticated users.To do so, the registration form should have a checkbox like “accept the privacy policy” (that describes how cookies are used), the user must check it, and then the website is free to set auth cookies.
- If a website wants to set tracking cookies for everyone.To do so legally, a website shows a modal “splash screen” for newcomers, and require them to agree for cookies. Then the website can set them and let people see the content. That can be disturbing for new visitors though. No one likes to see “must-click” modal splash screens instead of the content. But GDPR requires an explicit agreement.
GDPR is not only about cookies, it’s about other privacy-related issues too, but that’s too much beyond our scope.
Summary
document.cookie
provides access to cookiesCookie Format Converter
- write operations modify only cookies mentioned in it.
- name/value must be encoded.
- one cookie up to 4KB, 20+ cookies per site (depends on a browser).
Cookie options:
path=/
, by default current path, makes the cookie visible only under that path.domain=site.com
, by default a cookie is visible on current domain only, if set explicitly to the domain, makes the cookie visible on subdomains.expires
ormax-age
sets cookie expiration time, without them the cookie dies when the browser is closed.secure
makes the cookie HTTPS-only.samesite
forbids the browser to send the cookie with requests coming from outside the site, helps to prevent XSRF attacks.
Additionally:
- Third-party cookies may be forbidden by the browser, e.g. Safari does that by default.
- When setting a tracking cookie for EU citizens, GDPR requires to ask for permission.