What is the shortest function for reading a cookie by name in JavaScript?

Shorter, more reliable and more performant than the current best-voted answer:

const getCookieValue = (name) => (  document.cookie.match('(^|;)\\s*' + name + '\\s*=\\s*([^;]+)')?.pop() || '')

A performance comparison of various approaches is shown here:

http://jsperf.com/get-cookie-value-regex-vs-array-functions

Some notes on approach:

The regex approach is not only the fastest in most browsers, it yields the shortest function as well. Additionally it should be pointed out that according to the official spec (RFC 2109), the space after the semicolon which separates cookies in the document.cookie is optional and an argument could be made that it should not be relied upon. Additionally, whitespace is allowed before and after the equals sign (=) and an argument could be made that this potential whitespace should be factored into any reliable document.cookie parser. The regex above accounts for both of the above whitespace conditions.

This will only ever hit document.cookie ONE time. Every subsequent request will be instant.

(function(){    var cookies;    function readCookie(name,c,C,i){        if(cookies){ return cookies[name]; }        c = document.cookie.split('; ');        cookies = {};        for(i=c.length-1; i>=0; i--){           C = c[i].split('=');           cookies[C[0]] = C[1];        }        return cookies[name];    }    window.readCookie = readCookie; // or expose it however you want})();

I'm afraid there really isn't a faster way than this general logic unless you're free to use .forEach which is browser dependent (even then you're not saving that much)

Your own example slightly compressed to 120 bytes:

function read_cookie(k,r){return(r=RegExp('(^|; )'+encodeURIComponent(k)+'=([^;]*)').exec(document.cookie))?r[2]:null;}

You can get it to 110 bytes if you make it a 1-letter function name, 90 bytes if you drop the encodeURIComponent.

I've gotten it down to 73 bytes, but to be fair it's 82 bytes when named readCookie and 102 bytes when then adding encodeURIComponent:

function C(k){return(document.cookie.match('(^|; )'+k+'=([^;]*)')||0)[2]}

Assumptions

Based on the question, I believe some assumptions / requirements for this function include:

• It will be used as a library function, and so meant to be dropped into any codebase;
• As such, it will need to work in many different environments, i.e. work with legacy JS code, CMSes of various levels of quality, etc.;
• To inter-operate with code written by other people and/or code that you do not control, the function should not make any assumptions on how cookie names or values are encoded. Calling the function with a string "foo:bar[0]" should return a cookie (literally) named "foo:bar[0]";
• New cookies may be written and/or existing cookies modified at any point during lifetime of the page.

Under these assumptions, it's clear that encodeURIComponent / decodeURIComponent should not be used; doing so assumes that the code that set the cookie also encoded it using these functions.

The regular expression approach gets problematic if the cookie name can contain special characters. jQuery.cookie works around this issue by encoding the cookie name (actually both name and value) when storing a cookie, and decoding the name when retrieving a cookie. A regular expression solution is below.

Unless you're only reading cookies you control completely, it would also be advisable to read cookies from document.cookie directly and not cache the results, since there is no way to know if the cache is invalid without reading document.cookie again.

(While accessing and parsing document.cookies will be slightly slower than using a cache, it would not be as slow as reading other parts of the DOM, since cookies do not play a role in the DOM / render trees.)

Loop-based function

Here goes the Code Golf answer, based on PPK's (loop-based) function:

function readCookie(name) {    name += '=';    for (var ca = document.cookie.split(/;\s*/), i = ca.length - 1; i >= 0; i--)        if (!ca[i].indexOf(name))            return ca[i].replace(name, '');}

which when minified, comes to 128 characters (not counting the function name):

function readCookie(n){n+='=';for(var a=document.cookie.split(/;\s*/),i=a.length-1;i>=0;i--)if(!a[i].indexOf(n))return a[i].replace(n,'');}

Regular expression-based function

Update: If you really want a regular expression solution:

function readCookie(name) {    return (name = new RegExp('(?:^|;\\s*)' + ('' + name).replace(/[-[\]{}()*+?.,\\^$|#\s]/g, '\\$&') + '=([^;]*)').exec(document.cookie)) && name[1];}

This escapes any special characters in the cookie name before constructing the RegExp object. Minified, this comes to 134 characters (not counting the function name):

function readCookie(n){return(n=new RegExp('(?:^|;\\s*)'+(''+n).replace(/[-[\]{}()*+?.,\\^$|#\s]/g,'\\$&')+'=([^;]*)').exec(document.cookie))&&n[1];}

As Rudu and cwolves have pointed out in the comments, the regular-expression-escaping regex can be shortened by a few characters. I think it would be good to keep the escaping regex consistent (you may be using it elsewhere), but their suggestions are worth considering.

Notes

Both of these functions won't handle null or undefined, i.e. if there is a cookie named "null", readCookie(null) will return its value. If you need to handle this case, adapt the code accordingly.