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

Question

What is the shortest, accurate, and cross-browser compatible method for reading a cookie in JavaScript?

Very often, while building stand-alone scripts (where I can't have any outside dependencies), I find myself adding a function for reading cookies, and usually fall-back on the QuirksMode.org readCookie() method (280 bytes, 216 minified.)

function readCookie(name) {
    var nameEQ = name + "=";
    var ca = document.cookie.split(';');
    for(var i=0;i < ca.length;i++) {
        var c = ca[i];
        while (c.charAt(0)==' ') c = c.substring(1,c.length);
        if (c.indexOf(nameEQ) == 0) return c.substring(nameEQ.length,c.length);
    }
    return null;
}

It does the job, but its ugly, and adds quite a bit of bloat each time.

The method that jQuery.cookie uses something like this (modified, 165 bytes, 125 minified):

function read_cookie(key)
{
    var result;
    return (result = new RegExp('(?:^|; )' + encodeURIComponent(key) + '=([^;]*)').exec(document.cookie)) ? (result[1]) : null;
}

Note this is not a 'Code Golf' competition: I'm legitimately interested in reducing the size of my readCookie function, and in ensuring the solution I have is valid.

Answer 1

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.